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The reproductive biology and population dynamics of B. improvisus were investigated. This barnacle species prefers to settle close to its conspecifics (0.4-0.8 cm), but there were more isolated individuals of B. improvisus (up to 3 cm apart) found compared with E. modestus (not more than 1.6 cm apart). This could be the result of competition for space between these two barnacles or a consequence of the ability of B. improvisus to self-fertilise, which allows the species to reproduce without the requirement of neighbouring individuals. In southern England, the breeding season of B. improvisus was between March and October, and the putative period of planktonic larvae release was from July to August. Settlement of this barnacle occurred between July and September. The population genetic structure of this barnacle was investigated both at the local- (within the U.K. and Baltic) and broader- (between the U.K. and the Baltic) levels using mtDNA PCR-RFLP analysis of the COI-COIII genes. The results of the local-scale study suggest that the populations around the British Isles are isolated and that the level of gene flow between the estuaries is low. In contrast, the level of gene flow between the populations within the Baltic is high. This indicates that the planktonic larvae of this barnacle are capable of dispersing throughout the Baltic. In the broad-scale study, there was no significant genetic differentiation between the British and Baltic populations. This suggests that the level of gene flow between the two regions is high. Combination of the results from both local- and board-scale studies indicated that the populations of this barnacle species in the two regions could be derived from the same gene pool.