Inter-simple sequence repeats markers were used to determinate the genetic variability of Fasciola hepatica populations recovered from sheep and cattle from Spain (Sp1, Sp2, Sp3 and Sp4), UK (Eng), Ireland (Ir) and Mexico (Mex). Twenty five primers were tested but only five produced 39 reproducible bands, being 71·79% polymorphic bands. This percentage ranged from 10·26% in Sp4 to 48·72% in Sp1, and per host between 28·21 and 48·72% in sheep and between 10·26 and 38·46% in cattle. This relatively low range of genetic diversity within populations, with a mean of 34·40%, implies that a large proportion of variation resided among populations. The population differentiation (Gst = 0·547) indicated that 54·7% of variation is due to differences between populations and 45·3% due to differences within population. The Nei's distance ranged between 0·091 and 0·230 in sheep and between 0·150 and 0·337 in cattle. The genetic relationships between populations and individuals were shown by a UPGMA dendrogram and a principal coordinate analysis; both grouped all populations separately from Sp4, a population of from the Midwest of Spain with the lowest level of diversity. Small genetic distances were observed between Eng and Ir, on the one hand, and Sp1, Sp2, Sp3, from the Northwest of Spain, together with Mex, on the other.