Syngnathid fish (pipefish and seahorses) are unique among teleost fish in that their ovary consists of a rolled sheet with germinal ridge(s) on the dorsal side running along the entire length of the sheet. A distinct difference is seen in the ovarian structure between polygamous Syngnathus pipefish and monogamous seahorses (Hippocampus spp.), the former having one germinal ridge and the latter with two ridges. This study examined the ovarian structure and the mode of egg production in a monogamous pipefish Corythoichthys haematopterus. The ovary of C. haematopterus had two germinal ridges like that observed in monogamous seahorses. There were two distinct groups of follicles in the ovary, one being a cohort of extremely small follicles and the other a cohort of follicles developing and increasing in size with the passage of time. We suggest that the ovarian structure and the mode of egg production in this pipefish are adaptations to monogamy.