Invasive Ponto-Caspian gobies have recently caused dramatic changes in fish assemblage structures throughout the Danube basin. While their presence in the Croatian part of the basin has been noted and distribution studied, their dietary habits, impact on native fish communities, condition, age and growth, presence of haplotypes and origin, until now, have been unknown. In 2011, 17 locations in the Sava River Basin were sampled for fish and 15 for benthic invertebrates. Fish population monitoring data, available for nine seasons (2003-2006 and 2010-2014) and 12 locations, were used to analyse the impacts of Ponto-Caspian gobies on benthic fish abundance. Gut content analysis indicates that the monkey goby (Neogobius fluviatilis) diet is very diverse, but dominated by Trichoptera, Chironomidae, Bivalvia and Odonata. The diet overlaps considerably with the round goby (Neogobius melanostomus) diet, although Gastropoda are dominant in the latter's diet. Small fish and Gammarus sp. dominate the bighead goby (Ponticola kessleri) diet. Comparison of gut content with the prey available in the environment indicates that monkey and round gobies exhibit preference for Trichoptera, Megaloptera and Coleoptera, and bighead goby for Trichoptera, Gammarus sp. and Pisces. Ponto-Caspian gobies in the Sava River are spreading upstream, towards the reaches with lower fish diversity. Analyses indicate potentially positive impacts of PontoCaspian gobies presence on some fish populations: round and bighead goby on Balkan golden loach (Sabanejewia balcanica) and monkey goby on common carp (Cyprinus carpio), crucian carp (Carassius carassius), burbot (Lota lota) and Balkan loach (Cobitis elongata). However, there are also indications that bighead and round goby could adversely impact the native chub (Squalius cephalus) and zingel (Zingel streber) populations, respectively. Positive allometric growth has been determined for all three analyzed Ponto-Caspian goby species, suggesting that they have been present for a longer period of time. Condition factor, as a function of available energy resources, is the largest for the round goby and the lowest for the monkey goby. Age I, II, III have been determined for the monkey goby and age I, II, III, IV for the round and bighead goby, with the highest growth rate determined for the monkey goby and the lowest for the round goby. For each of the analyzed Ponto-Caspian goby species only one haplotype, originated from native Black Sea populations, has been found. Determined monkey goby haplotype has also been found in Ukraine and Slovakia and determined round goby haplotype in Ukraine, Bulgaria, Slovakia, Serbia, Poland and Belgium. Determined bighead goby haplotype has also been found only in Ukraine. A small number of haplotypes indicates a relatively small number of Ponto-Caspian goby populations and potentially reduces their invasive potential in the Sava River Basin. Although the PontoCaspian goby caused significant disruptions in some areas of the Danube basin, density of its population and impacts are relatively limited in the Sava River and its tributaries. As Ponto-Caspian gobies are still in the expansionary period of invasion and the ecosystem still adapting to new circumstances, continued monitoring of fish population dynamics in the Sava basin is needed to determine the outcome and impacts of this invasion.