The Hong Kong-originated medaka fish Oryzias curvinotus expresses nine genes (OcGC1?OcGC8 and OcGC-R2) for membrane guanylyl cyclases (membrane GCs) and three genes (OcGCS-?1, OcGCS-?2, and OcGCS-?1) for soluble GC subunits. The deduced amino acid sequences of membrane GCs expressed in O. curvinotus were quite similar to those expressed in the Japanese medaka Oryzias latipes, including a novel membrane GC gene, OlGC8, first isolated and characterized in O. latipes. O. curvinotus was able to produce hybrids with O. latipes, irrespective of the direction of crossing, and the resulting hybrids expressed both maternal and paternal soluble GC subunit genes, suggesting the possibility of the formation of a chimeric heterodimer in the hybrids. However, in early embryogenesis of hybrids, the maternal soluble GC subunit genes were expressed earlier than the paternal soluble GC subunit genes, suggesting that the maternal soluble GC subunit genes interact more effectively with maternal effector molecules such as transcription factors than with those of paternal origin.