Development of the cotylocidium within its egg has only been described for a limited number of trematodes in the Aspidogastridae. The objectives of this study were: (1) to determine the location of cotylocidium development within the egg for Cotylaspis insignis; and (2) to establish a time line for developmental stages of this larva within the egg. Unembryonated, ectolecithal eggs in the uterus of the adult worm showed a large, centrally located zygote surrounded by vitelline cells. Eggs shed within the host, Lampsilas radiata luteola, and eggs released into surrounding water from the host contained zygotes or embryos undergoing early (i.e., 3 blastomeres) cleavage. This indicated that subsequent cotylocidial development within the egg proceeded outside the host in the external environment. These subsequent developmental stages of the cotylocidium larvae within the egg are described over the next 36 d in vitro at 20°C, culminating with the hatching of a motile larva. Our findings are compared with those previously reported for other aspidogastrean species.