We studied impacts of the introduced small Indian mongoose Herpestes auropunctatus on the herpetofauna on six islands in the Adriatic Sea, Croatia, comparing abundances of reptiles and amphibians on three islands with the mongoose to those on three islands without the mongoose. We used four types of sampling surveys: distance-constrained surveys, visual encounter surveys, special searches and accidental trapping. The horned viper Vipera ammodytes and Balkan green lizard Lacerta trilineata were absent from two mongoose-infested islands (Korčula and Mljet) and rare on the third (Hvar); they were common only on the mongoose-free island where they had historically been present (Brač). The European green toad was absent from one mongoose-infested island, where it had historically been present and rare on the other two. It was common on two of the three mongoose-free islands. Other herpetofaunal species were either very scarce or completely absent on the three mongoose-infested islands. Most of these species also occur on the mainland but are already scarce there; some are strictly protected under Appendix II of the Berne Convention. The recent spread of the mongoose to the European mainland suggests the need for urgent control to protect vulnerable herpetofauna.