AbstractTwelve cotton strains, six glanded and six glandless, with different seed‐oil contents, were mated in a full diallel system. All genotypes were evaluated for seed‐oil percentage (SO), seed index (SI, mg/seed) and seed‐oil index (SOI, mg/seed) to obtain information on the inheritance of these traits and to assess the significance of maternal and reciprocal effects. Data generated from the diallel mating system were additionally divided into two full diallels, glanded and glandless, of six parents each and evaluated by combining ability and diallel analyses.The results indicated that maternal effects were not statistically significant for any trait, but reciprocal effects were significant for SI and SOI. Additive effects, or general combining ability (GCA), were highly significant in both analyses. Deviations from additivity, or specific combining ability (SCA), were significant for SI and SOI, and for SO in the glanded diallel. Deviations from additivity were not homogeneous over all the genotypes. Only the additive parameter of the genetic analysis for glandless SO was significant. This result indicated that additivity was greater in crosses involving glandless genotypes. Heritability of 0.53 based on GCA values was obtained for SO, which indicated that selection procedures could be applied successfully to change the oil content of cottonseed.The highest SO parent was glandless, and the glandless genotypic arrays averaged more SO than glanded arrays, indicating that glandless genotypes could be preferred over glanded in breeding for this trait.