In order to develop high-yielding cotton hybrids superior to the existing cotton varieties, an experiment was conducted during 1983 and 1984 in three locations (two in Mexico and one in U.S.A.) which had contrasting temperatures during the growing season. Materials consisted of 28 F, hybrids - involving nine parents of Gossypium hirsutum Linn. and two parents of Gossypium barbadense Linn. plus two checks (Stoneville 213 and Deltapine Acala 90). Highly significant interactions between locations and hybrids were found for the following variables: seed cotton yield, lint yield and plant height. Hybrids had low fertility and were low yielding at the hottest location (Ciudad Obregon, Sonora). Certain hybrids competed well with the checks at Matamoros Coahuila and Las Cruces, New Mexico. However, Deltapine 90 was the best yielder, overall.