The effect of low pH on growth and photosynthesis was compared in two strains of Chlorella isolated from symbiosis with Paramecium bursaria. One, 3N813A, releases large amounts of maltose at low pH; the other, NC64A, does not release maltose in culture. Maltose synthesis at low pH appeared to have a greater effect on cell growth than pH by itself. Growth of maltose-releasing 3N813A showed a direct relationship with pH; growth of non-maltose-releasing NC64A was reduced only as pH fell from 7·0 to 5·5. At pH 5·0, net photosynthetic capacity of 3N813A cells was reduced to 76% of controls at pH 7·0, but the specific growth constant (k) was reduced to 22%, and at pH 4·0 growth and nitrate uptake were inhibited. In 3N813A cells at pH 5·0, carbon (C) was diverted from cell growth, net chlorophyll synthesis, assimilation of nitrogen (N), and synthesis of protein and starch to maltose release, but C[ratio ]N ratios were unchanged. In NC54A cells, starch but not protein synthesis was inhibited at pH 5·0, and a reduction in accumulation of N led to an increase in C[ratio ]N ratios. Unlike N starvation, growth of 3N813A cells at pH 5·0 did not reduce chlorophyll content. When measured at pH 7·0 with excess bicarbonate, photosynthetic parameters of both 3N813A and NC64A cells grown at pH 5·0 were similar to those of cells grown at pH 7·0; however, net photosynthesis of 3N813A cells N starved at pH 7·0 was 22% of that of N-sufficient controls. In symbiosis, release of excess photosynthate as maltose may inhibit cell growth by inhibiting assimilation of inorganic N, but because C[ratio ]N ratios are unchanged, photosynthetic capacity of cells is not affected.