We made daily visits to 47 Eared Grebe Podiceps nigricollis nests in two colonies to determine the frequency of conspecific brood parasitism. Three criteria were used to detect parasitism: the appearance of two or more new eggs in a 24 hour period, new eggs after the host had ceased laying, and the disappearance of a marked egg with the simultaneous appearance of a new unmarked egg. We detected parasitism at 68% of nests in one colony, 14% of nests in the second colony, and 38% of nests in both colonies combined. The spatial dispersion of parasitic eggs among host nests within one colony did not differ from random expectation but, with respect to temporal dispersion, parasites were more likely to lay in host nests in the laying stage than expected by chance. Egg loss was more common at parasitized nests than unparasitized nests, but further studies are needed to confirm whether this is due to egg rejection by hosts. A comparison of clutch sizes in parasitized and unparasitized nests indicated that unparasitized nests never contained more that four eggs - supernormal clutches of five or more eggs thus indicate that parasitism has taken place and, if used as an assay for parasitism, would have identified two thirds of the cases of parasitism in our study. Applying this assay to clutch size information collected 40 years ago at a site close to our study area indicates that parasitism may be a regular feature of Eared Grebe reproduction.