This is the report on the fish mating experiment, which was carried out in space shuttle Columbia, in July of 1994. More than a year has passed by after the space experiment. However, we still have been carrying out some of the post-flight analyses, for example, on the microscopic examination of the various tissues of some space-originated fry, quantitative studies for the position and movement of fish on the video-tapes taken in space, and so on. Though such detailed studies are still going on, the main results of that space experiment have already become clear, which made me to publish this book. The full use of color pictures may help the reader to understand the text. November 1995 Kenichi Ijiri Principal Investigator of IML-2 Medaka experiment