Reduced fitness of immigrants from alternative environments is thought to be an important reproductive isolating barrier. Most studies evaluating the importance of the relative fitness of immigrants to speciation have focused on reduced survival of immigrants (i.e., immigrant inviability). However, variation in fecundity appears to have a greater impact on variation in fitness than does variation in viability, suggesting that reduced fecundity of immigrants could act as an important yet largely overlooked reproductive isolating barrier. Using a model and a survey of studies of local adaptation, we evaluate the relative strength of reduced immigrant viability and fecundity as potential causes of reproductive isolation. We found that reduced fecundity as compared to reduced viability as a reproductive isolating barrier should increase in importance as the relative costs of reproduction increase. Consistent with the elevated demands of reproduction reported in the literature, we found that reproductive isolation from reduced immigrant fecundity was of similar magnitude or greater than that from reduced immigrant viability, particularly in the early stages of speciation. These results suggest that the important role of differential fecundity in local adaptation extends to speciation.