Elminius modestus Darwin 1854 is a small, estuarine barnacle, until 1940 found only in New Zealand and southern Australia but now well established in Europe. Using electrophoresis, 22 enzymes were compared between 10 populations of E. modestus from Europe and three populations from the Antipodes. Fourteen enzymes gave consistent, scorable results at 19 loci, of which 10 were monomorphic. Contingency comparisons of allele frequencies at the polymorphic loci showed no significant difference within the European samples, within the Antipodean samplesand between the European and Antipodean samples. These results were discussed in the context ofboth the history of the spread of the species and its planktonic larval phase. It seems likely that the species spread to Australia by remote dispersal from New Zealand before 1832, and thatthe initial settlement in U.K. waters in the 1930s must have been large enough to establish thespecies successfully with a genome representative of that of the mother population, at least atthe loci examined.