Genetic relations among five lines of Stylosanthes guianensis were studied by means of a diallel cross. Two parents each of S. guianensis var. guianensis and of the 'tardio' group of var. guianensis, plus one parent of var. intermedia (cv. Oxley) were crossed in all combinations. Pollen fertility, seed set, and shoot length of F1 hybrids were used to measure genetic relations. Pollen fertility of the var. guianensis x tardio hybrids did not differ from pollen fertility of var. guianensis x var. guianensis and tardio x tardio hybrids. Fertility of hybrids of Oxley was considerably lower than that of hybrids of var. guianensis and tardio parents. While parents were found to differ in seed set, the data provided no evidence of a reduction in seed set in hybrids due to incompatibility between the var. guianensis and tardio accessions. No seeds were harvested from hybrids of Oxley. Heterosis for shoot lenqth was observed in all crosses between var. guianensis and tardio parents,- while only one of four hybrids of Oxley exceeded mid-parent shoot length.