The life history of the soldier crab Mictyris guinotae in Amparu Tidal Lagoon on Ishigaki Island, Japan was investigated through population measurements with detailed size classifications. Surveys and measurements were carried out at 15 sampling points every 2 weeks during the 4-month incubation season starting in December 2005, and twice in April and in June 2006 when juveniles started and finished settlement, respectively, in the lagoon. Supplementary measurements were conducted every month from December 2006 to May 2007 to corroborate the results of the first experiment and to examine the growth rate of the juvenile population.The results showed that (a) the peak of incubation was from mid-January to mid-February, and the season of juvenile settlement was April to early June, which means that the duration of larval life in the ocean was about 2 months; (b) juveniles were distributed widely in the lagoon, and adults clustered to the west and south of the lagoon where the bed sediment was relatively fine with high ignition loss; (c) the survival rates of adults and juveniles in 1 year were 0.73 and 0.17, respectively, and the mean longevity of adults was about 3.7 years; (d) reproduction efficiency from eggs to juveniles was about 0.0040, which is much smaller than the survival rate of juveniles as well as adults, which means that the propagation of M. guinotae is mainly controlled by the condition that the larvae experience in the ocean.