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Abstract
The mosquitofish, Gambusia holbrooki, is one of the few tele‐ostean fish still present in the polluted waters of the River Sarno. The specimens taken in this river had 48 chromosomes, all acro‐centric, gradually decreasing in length in both sexes. However, several (12%) female plates showed a chromosome with a unilateral gap often accompanied by a chromosome number reduced to 47, due to the presence of a biarmed element derived by centric fusion of two uniarmed elements. The nuclear organizer regions (NORs) were localized on two of the smallest chromosome pairs by Ag NOR banding. C banding + DAPI revealed centromer‐ic AT‐rich heterochromatin on all elements. The results also suggest a possible mutagenic and toxic activity of the substances present in the polluted waters of the River Sarno.