<p>The plasticity of parental care behavior has been reported in several species of substrate-spawning fish, where females show parental care when males disappear, even in species where males typically provide parental care. In this study, we investigated the plasticity of sex roles in egg-caring behavior in the monogamous goby, <i>Lubricogobius exiguus</i>, where pair dissolution occurs relatively frequently. When both sexes cohabited, the males exhibited egg-caring behavior. However, when one member of the pair disappeared, both sexes cared individually for the eggs. <i>Lubricogobius exiguus</i> exhibited larval dispersal behavior (LDB), in which the parent fish held newly hatched larvae in their mouths and released them outside the nests. In most pairs, both males and females exhibited LDB. Furthermore, LDB was observed in solitary individuals, regardless of the sex role of the previous pair. These results suggest that <i>L. exiguus</i> exhibits plasticity in sexual behavior related to parental care, depending on the characteristics of the mating system.</p>