The aggregation-attachment pheromone componentso-nitrophenol (ONP) and methyl salicylate (MS) in maleAmblyomma variegatum ticks appeared after three days of feeding on the host and reached high values after about six days. Variable quantities of 1.3-7.3 μg ONP and about 0.6 μg MS were present within ticks. ONP and MS were released at the high rates of 300-1800 ng/hr and 20-600 ng/hr per male tick, respectively. After a temporary decrease, males continued to emit at high rates after nearby attachment of females. InA. hebraeum, ONP showed a similar pattern, but with a delay of about a day. A male, which had fed during 14 days, contained about 2 μg and released 225-280 ng/hr. Emission in forcibly detached males of both species dropped rapidly to low levels of less than 10 ng/hr per tick. Host skin and tick feces in the vicinity of feeding males were pheromoneimpregnated. The very high emission rates are consistent with the observations that the pheromone is an important component of the host-location mechanism of conspecifics. ONP and MS are produced in the dermal glands type 2 associated with the ventrolateral cuticle.