The detection of genotoxic effects using in vitro cell systems can be extremely useful in risk assessment procedures. However, care should be taken in the extrapolation of in vitro results since, amongst other factors, established cell lines may deviate from the genetic characteristics of their species. In this work, the genetic similarities between the RTG-2 cell line and rainbow trout individuals (Oncorhynchus mykiss) from several fish farms have been studied by the RAPD technique. Results show a significant analogy in the band patterns obtained for both systems, up to 73% of the bands composing the fingerprint of the RTG-2 cell line were found in all the individuals analysed. The inter-population similarity index (Lynch, 1990), considering the RTG-2 cell line as a population, gives a value of 0.931 between both systems. The dendrogram constructed from all the individuals, considering the RTG-2 cell line as just another individual of a single population, showed that the genetic structure of the cell line was not different from those of the other individuals tested. The strong genetic similarity of both systems, together with the previously proven capability of the RAPD technique to detect genetic alterations caused in vitro by genotoxic agents, can be very useful in genetic ecotoxicological studies.