Terrestrial algae are a common component of urban ecosystems, but the factors that affect their distribution are poorly understood. Rosenvingiella radicans is a filamentous green alga widespread in Atlantic Europe, where it often produces green patches at the base of urban walls. The distribution of this alga has been analysed by Chi-Square Automatic Interaction Detection (CHAID) in two western European cities, Oviedo (Spain) and Galway (Ireland). The analysis considered five factors : type of habitat, substratum, orientation of the colonized surface, type of area and intervening space. The results indicated that the type of habitat was the most important factor, although intervening space and orientation of the surface were also influential factors. Overall, it is considered that these factors interacted to create local conditions of high humidity, suitable for the colonization of the alga. No major differences in distributional patterns were detected between the two cities. This was the first study in which the CHAID procedure was applied to algae and it is concluded that its use is worthy of consideration for other species and algal assemblages.