When juveniles of Carcinonemertes spp. are removed from male crabs and raised in the laboratory, a 50:50 sex ratio is obtained. Isolated females produce egg strings typical of Carcinonemertes, except that the embryos are haploid, with about 13 chromosomes per cell. Larvae develop at least to hatching, and some do hatch. Nearly all of the 146 females so raised produced egg strings. Larvae of all females raised in isolation were haploid while larvae of females put with males were diploid. Females from both U.S. west coast species, Carcinonemertes epialti and C. errans, and from three hosts, Cancer magister, C. antennarius, and Hemigrapsus oregonensis, produced these haploid larvae by parthenogenesis.