To elucidate the deuterium isotope effect (DIE) in pheromonal activity and to investigate the biosynthetic pathway of 4,8-dimethyldecanal (4,8-DMD; 1), the aggregation pheromone of the red flour beetle (Tribolium castaneum), deuterated analogues of 4,8-DMDs (2, 3, 4, and 5), were synthesized and their pheromonal activities were tested using a two-hole pitfall olfactometer. Although no apparent DIE was observed in their pheromonal activities, 4,8-DMD-1-d(1) (2) was less attractive than other analogues, which suggested that the bond distance between the formyl group of I and its receptor was critical in pheromone recognition by T. castaneum. Copyright (C) 2004 John Wiley Sons, Ltd.