Offspring size at birth is assumed to be an important determinant of initial success in a variety of organisms. For natural selection to be effective at modifying offspring size at birth in a given population, there must be some level of additive variation for birth size. In ovoviviparous fish, initial offspring size appears to be entirely under maternal control, and thus any differences in birth size among offspring within females is not due to selectable genetic variation. However, additive genetic variation may exist between females due to genetic differences in allocation patterns to their offspring. Such "grandfather effects" have been suggested for mosquitofish (Gambusia spp.), though actual estimates of heritability have not been previously published. Herein, we used a two generation full-sib/half-sib breeding design to estimate levels of additive genetic variation among "grandfathers" in a population of Gambusia holbrooki. Significant genetic variation was not detectable for fathers, as was previously suggested (Reznick, 1981). However, detectable genetic variation was found at the level of grandfathers, with an estimate of heritability of approximately 7%. This level of heritability is consistent with other estimates of life history characters in many other species, and suggests that there is sufficient additive genetic variation for this important life history trait in natural populations of mosquitofish.