Electrophoretic variability was examined in 15 natural populations of Salicornia europaea and Salicornia ramosissima, two nearly indistinguishable, diploid, annual plant species common in salt marshes in England and Europe. All of the 800 plants examined were homozygous at the 30 loci which were scored and the number of isozymes for each of the enzyme systems was the same in all individuals. The electrophoretic mobilities of 24 of the enzymes were identical in all plants. With respect to the gene loci coding the other six enzymes, only two classes of individuals were detected: a plant possessed either one set of alleles, or an alternate set for the sis loci. One electrophoretic pattern was found primarily in individuals from the upper levels of salt marshes, whereas the other pattern was common in plants from the lower levels. The two patterns are characteristic of Salicornia ramosissima and Salicornia europaea respectively. The electrophoretic evidence confirms the reproductive isolation of the species, indicates an apparent lack of genetic variability in both of them and simplifies the interpretation of the ecological differentiation.