Inhibition of flocculation by sugars was studied in 41 strains of Saccharomyces cerevisiae. Two distinct groupings were found. The first type, including all strains containing FLO 1, FLO 4, FLO 5, FLO 8 and TUP 1, were partially inhibited by mannose only. The second type (NewFLO) were completely inhibited by mannose, maltose, glucose and sucrose. Sugar inhibitions were reversible and were exacerbated by increased agitation. NewFLO strains were more sensitive to inhibition by inorganic salts than the FLO 1 type strains. This inhibition is probably chaotropic in nature. Most FLO 1 type strains are constitutive, expressing flocculation throughout growth, whereas, NewFLO strains are completely repressed by the presence of excess ammonium ions. It is proposed that this indicates two entirely distinct mechanisms of flocculation, probably involving a mannose-specific lectin in the FLO 1 type and a broad specificity lectin in the NewFLO type.