A B S T R A C T The population structure of the land crab Johngarthia lagostoma was studied on Ascension Island from 2005-2007, predominantly during the period of the seaward breeding migrations; approximately 4000 crabs were examined. Sex ratio varied according to location of sampling. Males predominated in permanent residential areas on high ground. Sexes were equal along migration routes to the sea. Females predominated in the breeding sites on the shore. Very few immature crabs (, 60 mm CW)

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... were found. Males had modal and maximum sizes of 100-109 mm and 120 mm CW, females of 90-99 mm and 110 mm, respectively. An aging population is indicated. Crabs , 40 mm CW were all dark coloured, and colour variation into yellow, purple and intermediate morphs developed between 40-50 mm CW. Overall colour distribution of the population was 85% yellow, 5% intermediate, and 10% purple. Yellow morphs were more abundant in females (89%) than males (81%). The predominance of yellow morphs may be an adaptation to reduce heat stress.