The genome sizes (GS) and AT contents of five species of Anguilliformes (Anguilla anguilla, Anguilla rostrata, Conger conger, Gymnothorax unicolor and Muraena helena) belonging to three families (Anguillidae, Congridae and Muraenidae) were determined by flow cytometry and densitometry. Differences in Gs between Anguillidae-Congridae (about 3 pg) and Muraenidae (about 5 pg) were found, which correlate with the relative amounts of AT base pairs (around 42-43% for Anguillidae-Congridae and 51-52% for Muraenidae). Correlation of these results to karyotype parameters testified to a close phylogenetic relationship between Anguillidae and Congridae.