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Four of nine stock cultures of Agrobacterium tested formed mixtures of two types of colonies, which were distinguished on plates of aniline blue medium because of the difference in their production of two exocellular polysaccharides. The ability of organisms from the white colonies of strain 1.~012665 to produce large amounts of water-soluble polysaccharide was unstable; when kept on nutrient agar slants or in inorganic salt solution the bacteria mutated spontaneously to yield blue colonies producing large amounts of curdlan-type polysaccharide instead. I N T R O D U C T I O NCurdlan is a P-glucan which is composed entirely of (I +3)-P-glucosidic linkages and it forms a firm, resilient gel when heated in aqueous suspension (Maeda et al., 1967; Harada, Misaki & Saito, 1968a; Saito, Misaki & Harada, 1968). This polymer was first found to be formed in high yield by a mutant strain ( I O C~K ) (Harada et al., 1966) of Alcaligenes faecalis var. myxogenes 10c3 which produces a water-soluble, acidic polysaccharide (succinoglucan) (Harada, I 965). Recently, we have investigated the distribution of curdlantype polysaccharide in micro-organisms by using the aniline blue method in which organisms forming curdlan-type polysaccharide are detected as blue colonies on aniline blue plates. In this way, some strains of Agrobacterium were found to produce curdlan-type polysaccharide with water-soluble polysaccharide (Nakanishi et al., I 976). In addition, some stock cultures of Agrobacterium formed mixtures of blue and white colonies on aniline blue plates ; these variant forms developed spontaneously under normal conditions for maintenance of bacteria on agar slants. The present paper describes studies on the instability of polysaccharide production in Agrobacterium. METHODSStrains. Seven strains of Agrobacterium radiobacter, one strain of Agrobacterium rhizogenes and one strain of Agrobacterium tumefaciens (see Table I