A marine reserve is defined as a subset of the area over which a fish stock is dispersed and closed to fishing. This paper investigates what will happen to fishing outside the marine reserve and to the stock size in the entire area as a result of establishing a marine reserve. Three regimes are compared: (i) open access to the entire area, (ii) open access to the area outside the marine reserve, and (iii) optimum fishing in the entire area. Two models are used: (i) a continuous-time model, and (ii) a discrete-time model, both using the logistic growth equation. Both models are deterministic equilibrium models. The conservation effect of a marine reserve is shown to be critically dependent on the size of the marine reserve and the migration rate of fish. A marine reserve will increase fishing costs and overcapitalization in the fishing industry, to the extent that it has any conservation effect on the stock, and in a seasonal fishery it will shorten the fishing season. For stocks with moderate to high migration rates, a marine reserve of a moderate size will have only a small conservation effect, compared with open access to the entire area inhabited by a stock. The higher the migration rate offish, the larger the marine reserve must be in order to achieve a given level of stock conservation. A marine reserve of an appropriate size would achieve the same conservation effect as optimum fishing, but with a smaller catch.