Principles of invasion biology are brought to bear on the question of whether the medfly is established in California. Since its first discovery in 1975, the pest has been captured in the Los Angeles Basin in nine separate years including every year from 1986 through 1990. The trend has become distinct--the intervals between captures are decreasing, the numbers captured are increasing, and the area over which they are detected is expanding. In addition, appearances are seasonal and captures in recent years have occurred in many of the same cities and neighborhoods where medflies were found several years before. Evidence suggests that the medfly may be established in the Los Angeles area and that previous eradication programs did not eradicate the medfly from California. It follows that detection, exclusion, and eradication protocols may need to be reexamined.