DAMP (3-(2,4-dinitroanilino)-3′amino-N-methyldipropylamine), which differentially accumulates in acidic compartments, was used to identify such compartments in Toxoplasma gondii tachyzoites at the electron microscope level. In both free tachyzoites and dividing intracellular parasites the only sites of DAMP accumulation were mature and forming rhoptries. No labelling of other secretory organelles (micronemes and dense granules), the ER, Golgi or any other membrane-bounded organelles or anything resembling a lysosomal system was observed. Labelling of the forming rhoptries was higher and more homogenous than in mature rhoptries in which labelling was confined to the expanded ends of each organelle. The acid pH-dependent accumulation of DAMP in the forming and mature rhoptries was blocked by ammonium chloride and monensin, reagents known to abolish intracellular pH gradients. Estimates of rhoptry pH, based on the level of DAMP accumulation, show that the intralumenal pH of forming rhoptries is more acidic (pH 5·5–3·5) than the mature rhoptries (pH 7·0–5·0).