Nereis succinea and Platynereis dumerilii (Annelida, Polychaeta) are broadcast spawners and reproduce semelparously. The final events in reproduction, swarming and spawning are co-ordinated by sex pheromones.A water-soluble fraction of crude oil, the volatile fraction (C9-C16) of EKO FISK crude oil was found to induce release of gametes in male nereids at levels <0.3 ppm.Using vacuum distillation, column chromatography, preparative GC and GC-MS analysis we showed that C(5)-alkylated benzenes were most potent in inducing sperm release, of those n-butyl-4-methylbenzene and 1,4-diethyl-2-methylbenzene were found to induce release of gametes at concentrations ≥4 nM. This threshold is lower than those reported for natural pheromones (nereithione: 60 nM, uric acid: 600 nM) but higher than background levels of aromatic compounds of 0.05 nM and below.Other oil fractions showed additional effects, blocking pheromone reception or narcotising and intoxicating animals. Part of these effects could be assigned to naphthalenes at levels down to approx. 320 nM. In the original mixtures, their action was modified or compensated by the presence of gamete release inducing alkylated benzenes. Other highly paralysing substances remained elusive.