Reproductive biology, population genetics and ecology of three sea anemone species in U. K. populations were described by examination of fresh and histological preparations, starch gel electrophoresis, and mapping techniques. Sagartia troglodytes is shown to consist of two distinct species, corresponding to the recognised varieties decorata (now S. troglodytes) and ornata (renamed Sagartia ornata - Holdsworth 1855). S. troglodytes employs obligately out-crossinoviparous gonochoric reproduction. Populations have maximal genotypic diversity, good fits to single-locus Hardy-Weinberg (H-W) predictions and high levels of gene flow between populations. S. ornata broods asexually produced actinulae. Populations are highly clonal. Extreme deviations from out-crossing predictions and extensive differentiation between populations suggest no gene flow occurs. S. ornata has a chromosome number double that of the other three species. S. ornata is concluded to be a parthenogenetic polyploid "morphospecies". Sagartia elegans employs a mixed reproductive strategy, combining local asexual propagation by pedal laceration with out-crossing oviparous gonochoric sexual reproduction. Local, short term recruitment is mainly by asexual expansion of established clones. Genotypic diversity in most populations is high despite the occurrence of large localised clones. Most populations exhibit deviations from H-W predictions, and significant differentiation between populations at all variable loci confirms the lack of short term success of sexual reproduction and gene flow. Cereus pedunculatus displays genetic differentiation between colour pattern "morphs", but this may result from a predominance of asexual propagation and sub-habitat differentiation between clones. Populations consist of females and simultaneous hermaphrodites, both of which brood parthenogenetically produced actinulae. Sexually produced young may also be brooded. All populations display deviations from H-W predictions, and extensive inter-population differentiation. The populations studied are concluded to be marginal in the species range, a move away from sexual reproduction resulting in genetic isolation of populations and consequent drift and founder effects. All four species display high levels of genetic variability. No association of genetic variability with clonal success, degree of habitat heterogeneity, environmental conditions or life-history characteristics is apparent.