Effects of phosphate fertilization on the physiological activity of arbuscular mycorrhizal infection were studied using fungal succinate dehydrogenase and alkaline phosphatase activities as enzyme markers. Two plants, soybean and pineapple, were used because of their different growth rates and response to phosphate. Total mycorrhizal infection, estimated by trypan blue staining, was reduced by phosphate fertilization of soybean and pineapple in P-sufficient soils. In a P-deficient soil, fungal infection in pineapple roots was not modified by fertilizer application. The level of total mycorrhizal infection was not related to plant growth. Fungal alkaline phosphatase staining and to a lesser extent succinate dehydrogenase activity showed a relationship with plant growth; the potential of this procedure for estimating endomycorrhizal infection as a marker of efficiency of the symbiosis is discussed.