Abstract Potential seed-yield of cowpea has not been evaluated at extremely high densities. The factors associated with high seed-yield under extremely high plant density were investigated with contrasting vegetable and dry-grain bush-type cowpeas during the summers of 1985 to 1987 at the University of California, Riverside. A split-plot experimental design was used, with cultivars as mainplots and plant densities as subplots. In 1985 and 1986, two vegetable cowpea cultivars (UCR193 and UCR206) were planted at 100, 133, 200 and 400 thousand plants ha−1 ( tph . In 1987, four dry-grain cowpea cultivars (CB5, 8043, 507 and 637) and the two vegetable-cowpea cultivars were planted at the extreme densities of 100 and 400 tph . UCR193 consistently produced greater seed yield than UCR206 at 400 tph , but their yields were similar at 100 to 200 tph . Seed yields of CB5, 637, and 8043 were significantly greater at 400 tph than at 100 tph by 60, 44 and 26%, respectively. In contrast, seed yield of 507 was 15% less at 400 tph than at 100 tph . At 100 tph the yields of the dry-grain cowpeas were not significantly different. Changes in plant density had little effect on apparent harvest indices ( ahi ) for both the vegetable and dry-grain cowpea cultivars. Cultivar ranking for ahi was relatively stable across densities and years. Seed yield of cultivars with high ahi (UCR193, CB5, 8043 and 637) were more responsive to increasing plant density than cultivars with low ahi (UCR206 and 507), and yield response was positively correlated with ahi (r = 0.69 and 0.94 with n = 6 at low and high plant density, respectively). Seed yield response to higher plant density was negatively correlated with vegetative vigor as measured by shoot dry-matter (r = −0.51, n = 6) or leaf-area index (r = −0.65, n = 6) at full bloom and low plant density. Cultivar differences in responsiveness to density were not associated with differences in plant morphology. The results demonstrate that seed yield of some bush-type cowpea cultivars can be substantially increased in intensive commercial production by increasing plant density from the currently used densities of 100–133 tph to 400 tph . Selection for high apparent harvest index may be useful during early phases of breeding programs, with plants sown at low densities when developing bush-type cowpea cultivars with potential for high productivity under extremely high plant density.