Environmental conditions and host features that influence the abundance of an exotic branchiobdellidan were studied. One of the most widespread invasive crayfish species in Europe is the signal crayfish, Pacifastacus leniusculus (Dana, 1825). Along with this invasive crayfish, other alien species that live on the crayfish¿s body surface, such as bacteria, algae, fungi and invertebrates, were introduced. One of these symbiont species is a branchiobdellidan worm, Xironogiton victoriensis (Gelder and Hall, 1990). Because a previous study identified this exotic branchiobdellidan in 3 different and spatially separate populations of the red swamp crayfish, Procambarus clarkii (Girard, 1852), an investigation into the factors affecting its abundance was necessary. We sampled 1817 sites between 2005 and 2013 in the Ebro Basin (Spain), and the distributions of X. victoriensis and P. leniusculus were represented and interpreted. Furthermore, we sampled 2914 crayfish from 43 Spanish localities to investigate the influence of environmental conditions and host features (sex, body size, body condition, density and ecdysis) on the abundance of X. victoriensis. The abundance of the branchiobdellidan was higher in the largest individuals and in those showing the best body condition. The crayfish body condition (or welfare) increased significantly with the branchiobdellidan infestation, suggesting that mutualism is the biological relationship between X. victoriensis and P. leniusculus. Although 91.04% of the adult signal crayfish were infested with X. victoriensis, only 59.3% of the juveniles were infested. Nonmolted crayfish had higher branchiobdellidan abundance than that of recently molted host specimens; however, the abundance of the symbiont did not differ between sexes and host densities. Finally, biochemical oxygen demand, phosphates, coliforms, and potassium were positively related with the branchiobdellidan abundance. The relationships between some environmental variables and the abundance of branchiobdellidans have not been previously well studied, and these findings will be important for risk assessments and models predicting the spread of exotic branchiobdellidans.