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Introduced species are one of the most important anthropogenic impacts on freshwater ecosystems with many direct and indirect effects on native taxa. Among other invasive groups, such as plants, mussels and fish, several alien Decapoda species have also spread successfully in Europe in the last 110 years. In Hungary three native (Astacus astacus, Astacus leptodactylus, Austropotamobius torrentium) and three alien Decapoda species, namely Orconectes limosus, Pacifastacus leniusculus and Eriocheir sinensis are known to be present. O. limosus, which had been tried for use in crayfish farming in the 1950s, was the first to occur in the country’s natural waters. Initially it was found in the Danube at river km 1,653 at Budapest in 1985. Since then, it has been spreading fast and populations have reached high abundances. By 1998, it was already in the Gemenc section of the river colonising five 50 km × 50 km UTM squares. In the early 2000s it was also found at Mohács (and further downstream in Croatia), in canals in the Great Hungarian Plain and in the River Ipoly, which added three new 50 km × 50 km UTM squares to its previously known distribution area in the Carpathian Basin. On the basis of the available records from the past 20 years, the downstream colonisation speed of this decapod was calculated to be more than 13 km yr–1, but if its presence at Kopácsi rét/Kopacki rit in Croatia is also taken into consideration, it is over 16 km yr–1. It is unknown, however, how much this process was helped by deliberate introductions, if at all. Besides the main watercourse of Hungary, O. limosus is also common in its lowland tributaries and spreading towards Lake Balaton along the Sió canal. However, it has not been recorded entering mountain streams in the Danube Bend, where A. torrentium lives, which is important for the conservation of that native species. If O. limosus spreads with the same speed and distribution pattern in the Carpathian Basin, it may colonise large rivers such as the River Tisza, their lowland tributaries and canals in the near future. Based on the present situation, O. limosus is likely to threaten A. astacus populations especially in the southern part of Transdanubia, perhaps leading to the elimination of some populations, but less likely to affect A. torrentium living in the mountains of the Danube Bend.