Reproductive ecology of the mudskipper Periophthalmus modestus was studied at Shinhamako Lagoon, inner part of Tokyo Bay, the northernmost habitat in Japan, in terms of conservation biology. Observation of courtship behavior and monthly capturing of fish to measure body length and weight were made from May to October 2004. Courtship behavior was recorded from June to August, indicating that spawning season ran this period. It took place at the low tide that occurred after 10: 00 A. M. and only at the part of mud flat near reed field. Body length of the females were significantly larger than that of the males. Condition factor decreased from June to July, and moderately increased until October. Energy preserved in body mass may be converted to reproductive activity after start of spawning period. Distribution of juveniles was observed weekly from July to September 2005. Most of the juveniles were observed near reed field, not at the open part of the mud flat. These results indicate that continuity between mud flat and reed field is important for the conservation of this species.
