The endobenthic deposit-feeder polychaetes of the Capitella capitata species complex have a strong relation with organically enriched sediments (OES). The effect of non-organically enriched sediments (NOES, 0.09% of organic content) and OES (6.26% of organic content) on mortality, growth and development of juveniles and adults of Capitella teleta (cultured for more than 40 years) and Capitella sp. from Alfacs Bay, Tarragona, Spain (acclimatized for one month) were analysed in laboratory conditions. Juveniles and adults were exposed (all individuals separately in different dishes) to the two treatments, periodically observed and measured until death. NOES considerably impaired survival, growth rates and development of juveniles and adults of both species, which showed highly variable negative growth rates under OES. Juvenile and adult C. teleta showed an improved maturity under OES, even producing two protandrous and one protogynous individuals, whereas no Capitella sp. reached maturity during the experiment.