The reproductive biology of Impatiens pallida Nuttal and Impatiens biflora Walt (Bal- saminaceae) was studied in relation to the evolutionary significance of cleistogamy and chasmogamy. Plant survivorship varied markedly between sites and was affected by abiotic agents such as drought and flooding, and occasional heavy damage from host-specific herbivores. Cleistogamy was the dom- inant mode of reproduction at all sites, but absolute and relative chasmogam production generally increased with total bud output. Extreme variability in seed production and reproductive mode was observed within and between sites. Cleistogams produced fewer seeds per ovary, matured faster and had a greater probability of surviving to produce seeds than chasmogams in all populations. The energetic investment in fertilization costs (i.e., sepals, petals, pollen and nectar) of chasmogams was > 100 times that of cleistogams. Calculation of energetic expenditure per seed based on survivorship curves for each flower type and energy investment for major developmental stages indicated chas- mogams were 2-3 times as costly as cleistogams. Greater chasmogam cost was also suggested by the earlier onset of senescence in greenhouse plants with high relative chasmogam output. Plants grown in the greenhouse under different light regimes displayed extreme plasticity in re- productive response as measured by absolute and relative output of cleistogams and chasmogams. The proportion of chasmogams produced per plant generally increased with light intensity. Plants collected from I. pallida populations where chasmogamy was rarely observed had significant chas- mogam production when grown in the greenhouse. The tremendous differences observed in plant survivorship and reproductive output demonstrate a high degree of temporal and spatial variation in quality of Impatiens habitats. It is suggested that these conditions have exerted strong selective pressure for reproductive plasticity. Multipurpose genotypes capable of producing cleistogams and chasmogams, depending on habitat suitability, are highly adaptive in environments where genetic tracking is inefficient. The evolution of a dual repro- ductive mode in Impatiens provides seed production at minimal expense via cleistogamy and the opportunity for outcrossing and pollen donation through chasmogamy. The outcrossing potential of Impatiens chasmogams is enhanced by (I) functional monoecy and (2) a fivefold difference in relative lifespans (J phase > Y), which results in strongly skewed intraplant sex ratios.