This work has determined that the major constraint to chickpea production in southern Queensland is a root rot disease caused by Phytophthora megasperma f. sp. medicaginis. A baiting technique was developed to determine inoculum potentials of Pmm in soil samples from chickpea fields. Lower estimates of inoculum potential were obtained when chickpea seedlings or excised parts thereof were used as baits instead of lucerne seedlings. There was no increase in lucerne bait sensitivity samples were air-dried and remoistened- prior to baiting. Baits removed shortly after the soil samples were flooded, a technique which reportedly reduces contamination by Pythium species, were less effective than those baits which were left in the flooded soil sample. More baits produced sporangia at 20°C than at 15°C. These factors were considered when a modified baiting technique, in combination with Most Probable Number ( MPN) analysis, was developed to determine levels of infective units of Pmm in soil samples from 11 chickpea fields in southern Queensland, collected during the 1988 growing season. Inoculum potentials of these field soils ranged from 0.01 to 10.95 infective units per gram oven-dry weight of soil, corresponding to levels previously reported for other Phytophthora-incited diseases. Pmm usually was not detected by baiting unless soil samples were collected from the vicinity of diseased plants, and inoculum potentials declined markedly in the absence of a growing chickpea crop. A glasshouse assay for resistance was developed using levels of inoculum that corresponded to the inoculum potentials encountered in naturally infested field soils. Infestation of potting mix at the rate of 13 oospores g-1 produced an MPN of 0.3 infective units g-1. This screening technique differentiated between a partially resistant chickpea cultivar (CPI56564) and a highly susceptible line (ICC6334), but failed to predict the responses of a wide range of cul ti vars in a field trial. The assay was improved by injecting a suspension of oospores into uni nfested pl anting mix in which chick pea seedlings were growing, and this provided a higher correlation between glasshouse and field results. Using this assay, there were no differences in response to the pathogen between seedlings aged 10 to 42 days. In the glasshouse tests, the disease was more severe at 20°C than at 30°C, and in treatments which involved continual or intermittent saturation of the soil rather than daily watering to field capacity. Different levels of zoospores were also tested as an inoculum source in a glasshouse assay for resistance, but variabil ity in plant response was higher than that encountered with oospores.The compound interest equation, fitted to cumulative proportions of diseased plants for 30 cultivars grown in soil naturally infested with Pmm in a field trial, was found to be the most suitable transformation to linearize data for most cultivars, when compared to the simple interest equation using coefficients of determination. Parameters which were useful in measuring the expression of resistance were the area under the disease progress curve ( AUDPC), the final disease incidence (FDI), and the apparent infection rate ( ₿1). The AUDPC was significantly correlated with both FDI and X0 (time from sowing until expression of first symptoms), and FDI was significantly correlated with ₿1. The most promising cultivars, other than CPI56564, which were considered resistant with respect to both FDI and ₿1, were CQ1591, ICC7568 and 232-4. Stomata located beneath the cotyledons of seven-day o1d chickpea seedlings were found to be an infection court for zoospores of Pmm during a histological study of the infection process. Zoospores accumulated in greatest numbers in the root hair zone. Penetration was usually intercellular, but intracellular penetration did occur, via infection pegs produced from swollen germtube tips. No differences in prepenetration or penetration events were found between a field resistant and a susceptible cultivar. The growth rate of hyphae of Pmm was higher in wounded root tissue, but not wounded epicotyl tissue, of the susceptible cultivar than in the resistant cultivar. Further work is needed to understand the population dynamics of the fungus in the presence and absence of the host. Future investigations should focus on elucidating mechanisms conferring resistance in the different cultivars, and determining the genetics of disease resistance in the different lines.