Intraspecific variation in Polydora ciliata (Johnston) was assessed on the basis of evidence from morphology and population genetics. Morphological characters were studied by scanning electron microscopy and differences were observed between populations of P. ciliata with respect to the setae of the enlarged 5th segment and in the length of caruncle. Gene frequencies were determined for all the nine populations of P. ciliata by the use of polyacrylamide slab gel electrophoresis. Seven enzyme systems comprising 13 loci were investigated from individual worms. The enzyme systems were: 1. acid phosphatase, 2. esterase, 3. glucose-6-phosphate dehydrogenase, 4. malate dehydrogenase, 5. malic enzyme, 6. phosphoglucose isomerase, and 7. xanthine dehydrogenase. The mean genetic identity and genetic distance were calculated using Nei's index. The genetic identity calculations revealed a high degree of similarity between populations of P. ciliata which bore in hard substrates. The 'boring' and 'non-boring' forms of P. ciliata were significantly different from each other with respect to phosphoglucose isomerase, esterase and acid phosphatase. Two other species of Polydora, P. ligni Webster and P. limicola (Annenkova) were also studied morphologically and electrophoretically in order to assess interspecific variation. P. ligni was found very similar genetically to the 'non-boring' form of P. ciliata. Other polychaetes which were studied electrophoretically include; 1. Arenicola marina, 2. Nereis diversicolor, 3. N. virens and N. fucata. Inter and intraspecific variation was observed but due to small sample sizes gene frequencies were not determined.<p
