[Heat resistance of aldolase of the hybrid embryos of sea urchins Strongylocentrotus droebachiensis and S, intermedius].  The time of expression of the genes controlling aldolase has been studied in the hybrid embryos female Strongylocentrotus droebachiensis X male S. intermedius. The enzyme heat resistance estimated by the temperature of 50% inactivation following the exposition for 30 min (T50) was used as its genetic marker. T50 of aldolase of the psychrophilic maternal species suffered practically no changes from the stage of mesenchyme blastula till the stage of 11 days old pluteus and equated 35.3 degrees. T50 of aldolase of autumn- and spring-spawning populations of the thermophilic paternal species equaled 39.5 degrees at the stage of 11 days old pluteus. The heat resistance of aldolase of the hybrid embryos did not differ reliably from that of maternal enzyme during the first 4 days of development (at 8 degrees) till the late gastrula stage and attained the maximum (T50 =36.9 degrees) on the 8th day (stage of pluteus). The expression of the genes controlling aldolase appears to take place between these developmental stages.