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Summary: Bacteria which were resistant to streptomycin and rose bengal were preferentially stimulated in the rhizospheres of several plant species as compared with those in the soil away from the rhizosphere. Leguminous plants were the most effective and clover plants as young as 6 days had some effect. The separate and combined effects of rose bengal and streptomycin in the isolation medium were examined. Rose bengal alone had little effect on bacterial numbers, whereas streptomycin alone decreased them. The two substances together affected rhizosphere numbers strikingly but inconsistently. The bacteria from rhizosphere and soil were grouped according to their nutritional requirements. Chromogenic bacteria with simple requirements were the most abundant forms resistant to streptomycin + rose bengal in the rhizosphere and a species of Flavobacterium was especially favoured.