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Ingestion of fluid by axenically cultured Tetra‐hymena pyriformis requires an inducer, which can be provided by peptone‐yeast extract broth. Cells in this medium contain many phagotrophic vacuoles; the formation of these is readily followed by allowing peptone‐yeast grown cells to ingest the impermeable dye, trypan blue. Tetrahymena grown in synthetic medium contain only few vacuoles and do not ingest trypan blue to any significant degree. Addition of peptone‐yeast medium to cells grown in a synthetic medium “induces” vacuolar formation and uptake of the dye. Calculations based on the rate of ingestion of trypan blue and of heat killed cells of Serratia marcescens indicate that the phagotrophic process cannot adequately supply the nutritional demands of the organism. Experiments using the colored amino acid analog, 3,5‐dinitrotyrosine contrast the rapid penetration of material through the cell membrane with the more sluggish route of phagotrophy.

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