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The freshwater hydra, Hydra viridis is normally associated with Chlorella-like, algal symbionts which inhabit the host's digestive cells. Under experimental conditions bleached hydra will reassociate with algae harvested from green hydra, but not from our cultures of wild type Chlorella or strain NC64A which when originally isolated from Paramecium bursaria was symbiotically competent. Because of its demonstrated selectivity, the reassociation process is hypothesized to involve a recognition interface whose active participants are the algae cell wall and the digestive cell membrane. The data presented here confirm the hypothesis and suggest some potential molecular characteristics of the interacting partners. Concanavalin A (Con A), a plant lectin, used widely for similar studies in other systems totally inhibits reassociation; Wheat Germ Agglutinin (WGA), ricin and Lens culinaris lectin do so to a lesser degree. These results are consistent with the hypothesis that glycoproteins on the cells' peripheries are involved in cell-cell recognition in this system.