Petitia occulta sp. n. provides an example demonstrating that a morphologically indistinguishable species taxon can be described by molecular markers alone. Phenotypically the polychaete belongs to Petitia amphophthalma Siewing, 1956, one of the numerous meiofaunal species that have previously been regarded as cosmopolitan. Genotypically, however, by means of randomly amplified polymorphic DNA (RAPD)–polymerase chain reaction (PCR) (a DNA‐fingerprinting technique) the population on the Indo‐Pacific island of Mahé can be identified as a distinct taxonomic entity as: (1) genetic distance values between the Petitia from the Seychelles and from other sites (Mediterranean, Canary Islands) are relatively great; (2) differently generated cluster analyses of genetic distances produce phenograms revealing a distinctly separate clade (bootstrap 100%) for the Seychelles animals; and (3) the Seychelles animals are characterized by eight monomorphic diagnostic (i.e. autapomorphic) DNA‐fragments. The RAPD procedure in this case requires the preparation of the whole organisms and, thus, no holotype is available. Instead the RAPD band‐pattern with the diagnostic markers of one individual obtained with six primers, isolated DNA of one individual, and syntypes in the form of fixed, complete individuals are deposited. Species differentiation is confirmed by genomic DNA sequencing (ITS2).