The freezing of spermatozoa is a technique used in aquaculture which permits the storage of excess spermatozoa from natural or induced spawning, and saves them for use in subsequent procedures. The cryopreservation process requires gradual, controlled lowering of temperature in order to reduce osmotic shock injury or ice crystal formation in the cells. This is difficult to accomplish in the field, where appropriate automatic equipment is not available. This study describes a portable manually operated device usable for accurately repeating required freezing protocols. The device includes a Dewar container for liquid nitrogen, a system of graduated pulleys, an acrylic sample carrier, and a low temperature thermocouple. To test the freezing system, pieces of male gonad of the scallop Argopecten purpuratus were frozen at different rates (1.1, 2.2, 4.6, 6.5, 8.8, 12.9, 19.2, 32.2 and 96.5°C·min-1). The cryoprotective mixture used was composed of lyophilized skim milk, ME 2SO, and sucrose, with a thawing rate of 573°C·min -1. The optimum freezing rate was 8.8°C·min-1, obtaining 51.3 ± 6.6% motile sperm. The mechanical freezer was efficient in cryopreservation of A.purpuratus spermatozoa at different freezing rates, since it permitted regulation of the lowering of the samples into the liquid nitrogen vapor at different speeds.