Little is known about direct interactions between diatoms. In this study, small diatoms attaching onto larger benthic and motile diatoms were considered. This phenomenon is commonly considered as epiphytism (with diatom epiphytes) although the exact nature of this interaction has not been investigated yet. The question that was addressed here is the specificity of the interaction, simply by observing species associations in their natural environment. A comparison of this special form of epiphytism was carried out between two sites, on a Mediterranean river and one of its tributaries. The results showed both host and epiphyte selection, but with some variability in species associations. This excludes the two alternative hypotheses that (i) large diatoms are a neutral substrate for smaller diatoms and that (ii) the interactions are strictly specific between two species.