A study of the morphology of Anastrepha ludens revealed three possible sites of pheromone secretion in the male. In males the salivary ducts branch out into numerous blind diverticulae and end in a large opaque ball of tubules. The pleural pouches of males consists of a row of columnar cells which extends bilaterally from the pleural margin of the abdomen throughout the length of the pouch area. In 2-day old males these cells are small with very little cytoplasm. These lengthen reaching a maximum size after 8 days. Two transparent sac-like structures were found associated with the male genitalia and are thought to be the third possible site of pheromone secretion. Comparisons of the behaviour of wild and laboratory-bred flies were caried out in a field cage. Differences were found in the distribution of the two fly types. The daily activity rhythms of feeding, resting and locomotion appeared to be similar. In sexual encounters there was no evidence that lab-bred males were less successful in defending their territories. There was an apparent lack of synchrony in the time and site of lek formation between the two types of calling males. 71% of the matings took place between lab-bred flies. For wild males, large size was an advantage in territorial encounters and in finding a mate but not for lab-bred males. The behaviour and changes in spatial distribution of released, sterile lab-bred flies on a host orange tree in the field was studied. The fly population in the tree declined rapidly. The emigration rate was lower on hot sunny days than on cool cloudy days. In the early morning, when it was cool the flies moved to the upper lighter parts of the tree. When the ambient temperature was higher than approximately 25°C the flies moved into the shadier parts of the tree. The daily activity pattern was similar to that observed in the field cage. Ripe oranges were highly attractive sources. The duration of flight and lift production by individuals of wild and laboratory populations of A. ludens was studied. Lab-bred flies flew longer and more readily than wild flies. Females flew for longer periods than males. Time of day did not affect flight duration in females or in lab-bred males but did affect flight duration in wild males. Flights in the evenings (after 1800hr) lasted longer than those in the mornings. Flight duration increased significantly with age for each fly type. Lift production varied with age and during continuous flight. Life increased with age with a corresponding increase in flight duration. During continuous flight, life decreased rapidly (ca. 40%) 1-2 minutes after the initiation of flight, thereafter decreasing more slowly. The results indicate that both wild and lab-bred flies have the capacity for long range migration.