The objective of our study was to determine the phylogenetic relationship among diploid and tetraploid species of cotton. Results indicated monomorphic banding patterns for ACO, PGM, and SKDH isozyme between Gossypium hirsutum and G. lanceolatum. Variation existed between these two species for MDH in the mobility of only a single band. This data indicated a very close phylogenetic relationship between G. hirsutum and G. lanceolatum. Manomorphic banding patterns were also observed for MDH and ACO isozymes between G. barbadense and G. darwinii. These two species differed only in PGR and SKDH isozyme banding patterns. Results indicated that G. darwinii may be classified as a subspecies of G. barbadense. G. mustelinum had banding patterns different from the other tetraploid species of cotton for PGM, MDH, and ACO isozymes. The results indicated G. mustelinum is a distinct species. Monomorphic banding patterns were observed for PGI isozymes among diploid and tetraploid species of Gossypium. Mobility of certain bands of PGM, MDH, ACO, and SKDH isozymes were identical among the diploid and tetraploid species screened. Isozyme data on PGM indicated that the D genome species screened can be classified into two groups. G. raimondii differed extensively from other species of the D genome in terms of PGM, SKDH, and ACO isozyme profiles. G. aboreum and G. herbaceum were monomorphic for banding patterns of PGM and SKDH isozymes. These two species differed from each other by only a single band in profiles of many ACO and MDH bands. These results indicate that relatively little variation exists between these two species of the A genome. The profiles for PGM are consistent with the hypothesis that A and D genomes are ancestral genomes of the tetraploid species of cotton. Overall results are not discordant with a hypothesis of polypbyletic origins for the tetraploid cotton species.