In a sample of water from each of two water troughs used by horses at the Frances Simes Hastings Natural History Reservation, Monterey County, California, many specimens of Vorticella were found with the macronucleus parasitized by bacteria. By comparison with the taxonomic accounts by Noland and Finley (1931) and Kahl (1935) the species of ciliate seemed to be V. simzilis Stokes. I am indebted to Dr. J. M. Linsdale, Director of the Reservation, for the collections of water containing the ciliates. The names of the troughs in the following paragraph are those used by him in the geographical records of the Reservation. The first sample was obtained from Upper Barn Trough on February 5, 1941. Upon examination of floated coverglasses on the following day I found numerous specimens of Vorticella, and all of them had parasitized macronuclei. Those observed on the next day also were all parasitized, but the number had greatly decreased. The ciliate was present in a collection made from the same trough on February 13, but no nuclear parasites were found. The second sample with parasitized Vorticella was obtained from Cow Pasture Trough on March 29, 1941; in this material there were also many unparasitized specimens. In the living ciliates, the hypertrophied nuclei provided an indication of the presence of the parasites, which on close examination could easily be seen as small refractile rods (Fig. 3). One specimen with a parasitized nucleus was seen in fission, with the macronucleus in a dumb-bell form. Fission was completed while the specimen was watched. Detailed studies of the parasite were made in Schaudinn-fixed, iron haematoxylin-stained preparations. When not parasitized, the greatly elongated macronucleus has a varying width that often is about 4-5 p. Nuclei only a little thickened, and with few scattered parasites, were seen. In many instances the parasitized macronucleus had a width of 10, 13, or more microns, usually retaining its elongated form. At an extreme of hypertrophy, it had become a large ellipsoidal body measuring about te lnthe length of the ciliate and i of its width. In one specimen the parasitized macronucleus had fragmented into two rounded bodies, one with a diameter of 23 p and the other of 15 p. There is a marked alteration of the substance of the macronucleus even when the parasites are present in small numbers. Nuclei not parasitized contain numerous close-set, deep-staining, relatively large granules