Early gastrula stage embryos of the purple starfish, Pisaster ochraceus, were raised for 10 days in microgravity (µG) in an Aquatic Research Facility aboard the space shuttle Endeavour (STS 77). Controls consisted of embryos raised at 1 × g (1G) in flight and embryos raised at 1G on the ground. Experimental organisms and controls were fixed on mission days (MD) 3, 4, 5, 6, and 7 and one sample was returned alive. Comparison of the µG embryos with the 1G in-flight controls and ground controls suggests that there is little difference in size and overall development. Scanning electron microscopic examination of the development of the esophageal musculature showed that the pattern of development and differentiation was normal and was the same in both the in-flight and ground controls. The esophageal muscle cells of specimens returned alive after 10 days in µG contracted normally. Detailed transmission electron microscopic examination of MD 7 embryos revealed a decreased amount of sarcoplasmic reticulum in the µG embryos compared with both MD 7 1G in-flight and ground controls. These results suggest that while exposure to µG may slow muscle differentiation slightly, it has little overall effect on embryos/larvae of up to 7-8 days of development.