Samples of the mud shrimp Callianassa subterranea (Montagu) were taken at irregular intervals (September and October 1989, April and July 1990, August 1991) from a depth of 47 m at a fixed station in the North Sea (54°35′N 04°50′E). At this site, mud shrimps appear to have a contagious distribution, the mean density varied between 38 and 59 individuals m-2 and the sex ratio was biased significantly to males (mean male: female ratio, 1·9), except for shrimps of ≥8 mm carapace length (CL) which had an equal sex ratio. The combined samples for the full sampling period showed a bimodal distribution for males and females, with a possible third mode for males, suggesting a life cycle of between 2 and 3 years. Only females ≥7 mm CL carried eggs. Lack of samples over winter prevent any conclusive description of seasonal population patterns; however, reproduction (based on the presence of ovigerous females) extended from April to September with a peak in July and post-larval shrimps (<3 mm CL) were present each month. The allometric growth of the primary chela identified male (6·5 mm CL) and female (7·5 mm CL) size at maturity. Differences in allometric growth after maturity result in males having larger and more massive primary chelae than females of equal carapace length. The parasitic isopod Ione thoracica (Montagu) infected between 20% (October) and 10–8% (August) of mud shrimps, with males having a significantly higher level of infestation than females.