ABSTRACTThis study tests the hypotheses that substrate stability and rugose microtopography are the main factors that determine the tissue‐specific epiphytism in the chlorophyteSporocladopsis novae‐zelandiaeChapman. Bothin vitroand field experiments showed that the epiphyte did not develop on stable algal surfaces nor on artificial (smooth or rugose) substrata. In field experiments, however, other macroalgae settled more abundantly on rugose substrata. Similarly, our field experiments did not support the hypothesis that the physical environment provided by the spatial location of the host in the intertidal or subtidal was a relevant factor associated with specificity. In contrast, our laboratory experiments suggested that some component of the host cell wall might trigger the formation of penetrating rhizoids. These did not develop in isolated cultures of the epiphyte but were detected when the epiphyte contacted its natural host. Also, preliminary results showed that rhizoid‐like structures developed, although at very low frequency, only in plants grown in culture medium containing a crude extract of sorus cortical t