Otolith microstructure of six Mediterranean fish species (Alepocephalus rostratus, Cataetyx alleni, Bathypterois mediterraneus, Coryphaenoides mediterranea, Coryphaenoides guentheri, Lepidion lepidion) is described. Fishes were caught in July 1988 between 1000 and 2300 m depth in the northwestern Mediterranean. Rhythmic growth patterns similar to daily growth rings and annuli were found. The von Bertalanffy growth curve was fitted to the age-length relationship derived from otolith interpretation as a first attempt to provide growth data for deep sea species in the northwestern Mediterranean. The growth parameters obtained were validated for one species by means of length frequency analysis. The growth rhythms responsible for the ring formation as well as growth rates in deep-sea fish are discussed.