1. Zoosporogenesis and zoospore structure of Lagenidium callinectes, parasitic on ova of the blue crab, Callinectes sapidus, are studied by electron microscopy. 2. The cytoplasmic thread connecting discharging cytoplasmic units appears as a simple constricted area of cytoplasm, containing much membranous material, and bound by only the plasma membrane. 3. Cleavage of discharged cytoplasm may be grouped into four stages and occurs by the coalescence of cleavage vacuoles. 4. During cleavage, microtubules contained within packets formed from the endoplasmic reticulum attach extracellularly to the anterior flagellum. 5. The zoospores are rather “unspecialized” with the two flagella arising from a raised area of the diagonal groove. 6. Rootlets interconnect the basal bodies and project anteriorly and posteriorly along the plasma membrane of the spore. 7. Microtubules project away from the basal bodies and appear to ensheath the beaked nucleus.