The gall fly Cecidochares connexa (Macquart) (Diptera: Tephritidae) was imported from Indonesia into Guam in 1998. It was field-established at one location in north central Guam, and its spatial and temporal spread in Guam was monitored. The biology of the gall fly is presented. No relationship between the number of larvae in a gall and the dimensions of the gall (length and width) existed. Three pupae per gall was the most frequent density with a range of 1–7. The sex ratio of flies emerging from galls was 1:1 irrespective of number of larvae per gall. C. connexa-induced galls established a “nutrient sink” as a result of the formation of a nutritive tissue along the walls of the larval chamber. Younger galls contained more mineral nutrients, namely, calcium, iron, potassium, manganese, and zinc, which were depleted as the galls matured, but the reverse trend was noted for sodium. Galls developing on terminal meristems significantly reduced the length of the shoots of Chromolaena odorata (Asteraceae).