Goussia girellae n. sp. is described from the opaleye fish, Girella nigricans. Merogonic stages were observed in the apices of intestinal epithelial cells, in the lamina propria, and in extraintestinal sites including liver, gills, and spleen. Gamonts were observed in the intestinal epithelial cells. Only unsporulated oocysts were detected in the intestine, and sporulation occurred when feces containing oocysts were incubated for 48 h in seawater at 21 deg C. Oocysts are elongated (24.8 x 14.7 micrometers) with a wall about 200 nm thick and have no residuum, micropyle, or polar granule. Sporocysts are ellipsoid, (8.5 x 4.5 micrometers) have a thin two-layered wall approximately 30 nm thick, and consist of two valves joined by a suture. (Copyright (c) 1988 by The Society of Protozoologists.)