Capitella capitata was formerly regarded as an excellent cosmopolitan indicator species for marine pollution or environmental disturbance. Following an oil spill in West Falmouth, Massachusetts in September 1969, when most of the benthic marine fauna was killed, the subsequent responses of a number of polychaete and other invertebrate species allowed us to rank species in order of decreasing opportunism: 1. Capitella capitata 2. Polydora ligni 3. Syllides verrilli 4. Micropthalmus aberrans 5. Streblospio benedicti 6. Mediomastus ambiseta. These polychaete species all showed the ability to increase rapidly, large population size, early maturation and high mortality that are characteristics of opportunists. Preliminary electrophoretic studies on two Mdh (malate dehydrogenase) loci in Capitella collected in the oil spill area and in adjacent control areas indicated short-term selection at the Mdh-2 locus (Grassle and Grassle, 1974).