To investigate the extent of protandric simultaneous hermaphroditism (PSH) in the genus Lysmata, observations were made on the sexual system of L. californica, a species from the warm-temperate eastern Pacific. Observations on a large number of female-phase individuals (FPs) from a museum collection indicated that FPs with broods containing advanced embryos spawn again (successively) soon after brood hatching. Female-phase individuals maintained in pairs went through successive cycles of embryo brooding concomitant with gonadal vitellogenesis, hatching, molting, and spawning. Time-lapse video observations confirmed that FPs are able to copulate as males and inseminate postmolt, prespawning FPs. Prespawning FPs maintained alone did not produce successful broods of embryos. Thus, FPs of L. californica are outcrossing simultaneous hermaphrodites. Size-frequency distributions of sexual phases showed that individuals develop first in the male phase (MP) and then change to the female phase. Individuals apparently change from MP to FP in a single molt, as shown by (a) “transitional” individuals that were fully MP externally but whose ovotestes were full of vitellogenic oocytes and (b) a distinct change in the presence/absence and growth of pleopod characters from MP to FP. Female-phase individuals retain male gonopores from the MP phase, and relative growth of male ejaculatory ducts is similar in MPs and FPs. Pleopod flanges related to embryo attachment and incubation are somewhat masculinized in FPs of L. californica. The sexual system of L. californica is similar to that of L. wurdemanni from the Gulf of Mexico and L. amboinensis from the Indo-West Pacific. Protandric simultaneous hermaphroditism is probably widespread if not ubiquitous in the genus Lysmata.