Use of mark-recapture techniques to estimate the size of fish populations relies on several assumptions, one being that marked and unmarked fish have equal probabilities of capture. This requires that marked fish, if released at their site of capture, mix randomly with unmarked fish in the population before recapture. We tested whether marked fathead minnows (Pimephales promelas) in four wetlands mixed randomly 1) when released at the capture site and 2) when transported and released in the center of the wetland. We captured and marked fish at two locations on opposite sides of each wetland. We recaptured fish three weeks later at these two original capture stations and at two additional sites spaced equal distances from the two capture stations. Results using fish released at their original capture location and those released in the center of the wetland indicated no difference between the observed and the expected number recaptured, with the expectation being equal numbers caught at each station. Our results indicate that researchers using mark-recapture methods to estimate fathead minnow population size may release marked fish at the site of capture and obtain accurate estimates.