Although species with both male and female sexual functions are often dichotomized into simultaneous and sequential hermaphrodites, many simultaneous hermaphrodites also exhibit sequential changes in sex allocation. In a field experiment using one such species, the gobiid fish, Lythrypnus dalli, female-biased individuals reallocated to male function in relation to their relative body size: consistent with the sizeadvantage hypothesis, large females were more likely to reallocate and large fish had the highest spawningrates. Individuals, despite internal allocation to both sexual functions, adopted only one behavioral gender. Behavioral males had higher reproductive rates than behavioral females, and laboratory experiments showed that females preferred to mate with large males. Behavioral males grew more rapidly and did notdiffer from behavioral females in survivorship. In addition, individuals who adopted male behavior but did not receive eggs in their nests maintained high levels of female tissue, whereas males that received eggs did not. Laboratory experiments showed that, unlike most hermaphroditic animals, L. dalli canchange allocation either from ‘female’ to ‘male’ or from ‘male’ to ‘female’. Thus, L. dalli shares haracteristics of both sequential and simultaneous hermaphrodites. Simultaneous hermaphroditism maybe maintained, in this species, to facilitate rapid sex change from female to male and to retain flexibility o that unsuccessful males can revert to reproduction as females.