Recent technological advances have created renewed interest in the development and production of hybrid cotton seed.

The present study was conducted to determine the levels of heterosis and the magnitudes of general and specific combining abilities in crosses between cytoplasmic male sterile lines derived from relatively modern cotton cultivars, and male lines representing the major sources of fertility restorers now publicly available to cotton breeders.

Incomplete sets of crosses involving 3 male and 13 female lines were grown in replicated trials at three locations. Estimates of heterosis, analyses of variance and combining ability estimates are presented for lint yield, and for- fiber length, strength and micronaire

Positive estimates of heterosis for lint yield ranged from 100 to 133% over high parents. Maximum observed heterotic responses over high parent were 109% for fiber length, 104% for fiber micronaire and 106% for fiber strength.

Statisically significant male x female interactions were observed for yield, fiber length and fiber micronaire indicating the presence of significant nonadditive genetic effects within the group of crosses studied for these traits. Observed male x female interactions for fiber strength, however, were nonsignificant indicating a predominantly additive genetic component.

The consistent nonsignificance in lint yield for the estimated location x males x females interactions is interpreted as an indication of potentially increased environmental stability in some F₁ hybrids.

Previously reported levels of heterosis for the traits studied are apparently repeatable in crosses involving modern Upland cotton cultivars.