A 2% solution of the disodium salt of 2,7-dibrom-4-hydroxy-mercurifluorescein (mercurochrome)-was used to stain malarial oocysts in temporary preparations by adding a small drop of the dye to the drop of physiological saline into which the midgut of the mosquito host was dissected. Guts stained in this manner presented oocysts stained red without marked staining of the mosquito tissue. Counts of oocysts could be made more quickly and accurately than without staining, and lower magnifications could be used for examination.