Effects of temperature, age, and photoperiod on pheromone titer in the pheromone gland of gypsy moth,Lymantria dispar(L.), were studied with the aid of gas chromatography. In laboratory-reared females, daily rhythmic changes in pheromone content were observed with peaks occurring toward the evening and increasing in quantity with female age. In wild females held in outdoor cages under very hot afternoon conditions, maximum pheromone titer was observed in the early morning and minimum titer in the afternoon. However, pheromone-oriented flight activity of the wild males was lowest in the morning and highest in the afternoon. Temperatures of 33 and 35°C caused decline in pheromone titer to near zero in laboratory-reared and wild females, respectively. Calling behavior was not dependent on the presence of pheromone in the gland; it occurred in newly emerged females before pheromone could be detected and in females held at temperatures that inhibited pheromone production.