Very little is known about the physiological interactions between plant hosts and symptomless endophytic fungi despite their widespread occurrence. We investigated the impact of two such fungi, Colletotrichum musae and Fusarium moniliforme, upon the photosynthetic capacity of two crop plants, banana and maize, respectively. Endophyte-free plants were obtained first and then infected with the fungi. Measurements of total chlorophyll content revealed very little difference between endophyte-free and infected plants of banana, whereas in maize they showed 50% reductions in the endophyte-infected plants. The maximum photochemical capacity (Fv/Fm ) was measured in order to determine if the plants had any photoinhibitory effect caused by biotic or abiotic factors. After 45 d of growth, endophyte-free banana plants had similar values of Fv/Fm to plants typical of nonstressed conditions, whereas the endophyte-infected plants showed a reduction of approx. 15%. Unlike banana, infected maize plants displayed values of Fv/Fm similar to those of control and endophyte-free plants, indicating that the maximum photochemical capacity was not affected by infection. The light response curves of both species showed that the photosynthethic capacity was severely reduced in endophyte-infected plants, reaching saturation at c. 400 μmol m−2s−1 whereas the control and endophyte-free plants were saturated at much higher photon flux densities. In banana the effect seemed to be due to an impairment of electron transport in the thylakoid membranes. By contrast, reduction of the photosynthetic capacity in maize was due to a reduction in chlorophyll content, leading to a decrease in the electron transport components and a consequent reduction in carbohydrate synthesis. It is possible that the reduction in the maximum yield of photosynthesis in both crops was caused by toxins produced by the fungi. Nevertheless there were no major macroscopic effects on the plants to indicate disease symptoms.