During June and July, 1992, 209 anglers were interviewed at Piledriver Slough to estimate the source of fishing effort. Responses were used to classify fishing effort as new effort expended because of stocking rainbow trout Oncorhynchus mykiss (63%), base effort expended regardless of stocking (6%), or transferred effort from other fisheries (31%). Relative magnitudes of new, base, and transferred effort estimated from anglers interviewed on weekdays, on weekends, or within a day after stocking were not meaningfully different. Transferred effort came from other stocked fisheries and from fisheries on resident species. Almost none of the fishing effort at Piledriver Slough was transferred from other fisheries because of regulations imposed on those other fisheries. Fishing parties with minors were more prevalent on weekends than at other times of the week.