Combining ability for a number of yield and quality attributes of cultivated cotton (Gossypium) was studied in two diallel experiments under high-yielding conditions in northern Australia. The first diallel comprised nine G. hirsutum parents of North American and African origin, the second diallel of thirteen parents besides American and African cultivars also included an Indian line, some Australian lines and an American G. barbadense cultivar. Each set of hybrids was grown as a half-diallel (plus the parents), first as a plant crop and then as a ratoon crop in the following season. Generally variation due to general combining ability (GCA) was significant and substantial for all traits, although specific combining ability (SCA) was also significant. GCA was shown to be closely associated with parental performance per se which may, therefore, be used with good reliability in choosing parents for use in a breeding program. The plant and ratoon performances were very similar in most instances. The African cultivars together with some medium-quality American ones showed high combining ability for yield, but the best GCA for quality characters was found in the high- quality parents. The G. barbadense cultivar was exceptional in this regard, hut it had a low GCA for yield, and there are other technical problems associated in establishing good inbred lines from hybrids between the two cultivated tetraploid species. Genotypic correlations between a pair of traits were usually similar for the plant and its respective ratoon crop, but often quite different for the two diallel combinations. In both populations selection for increased lint yield would tend to be accompanied by adverse changes in some other important characteristics.