Freshly fertilized eggs of brook trout, Salvelinus fontinalis, were cold-shocked, after 1 minute of water hardening, in a 5% glycerol solution at −1.5 C for 2 hours. Egg mortality was significantly increased by this process. A chromosome analysis indicated that 33% of the brook trout hatched from cold-shocked eggs from females induced to spawn were mosaic polyploids, compared with 12% of the fry from noninduced females. No exclusively triploid brook trout were found by chromosome analysis. Frequency distributions of erythrocyte nuclear volumes indicated a higher incidence of mosaic polyploidy among fish than did chromosome counts for the same specimens. The reasons for this inconsistency are not known.