This paper examines the demographic consequences of planktotrophic and lecithotrophic development in an estuarine polychaete. Two strains of Streblospio benedicti (Spionidae) were reared in the laboratory from birth through death at 20°C and salinity 34%. Survivorship and reproductive data were collected weekly and were used to construct life tables and population projection matrices for each development mode. Planktotrophic females reproduced earlier, and had higher fecundity and a shorter generation time than lecithotrophic females, but also exhibited higher mortality early in life. Despite the apparently opportunistic nature of the planktotrophic life history traits, the finite rate of increase (l) in the lecithotrophic strain (1.319 wk ) exceeded that of the planktotrophic strain (1.205 wk ). Net reproductive rate (R ) was also higher for the lecithotrophs (93.4) than for the planktotrophs (17.6) Peak reproductive values were attained earlier in planktotrophs than in lecithotrophs. Sensitivity analyses indicate that l is most sensitive to changes in larval and juvenile survivorship, and that the differences in l were almost completely determined by life table differences during the first 15 wk of life. The potential population growth rates obtained in this study agree well with those estimated for other opportunistic polychaete species such as Capitella sp. I and Polydora ligni. Under uniform conditions the two strains of S. benedicti achieved similar growth rates with very different life history traits. We hypothesize that each combination of traits may be adaptive under different circumstances in the field.