The paper deals with the detection of a passive sonar target in the presence of ambient noise and a plane‐wave interference. Target signal, interference, and ambient noise are assumed to be Gaussian random processes, the ambient noise being statistically independent from hydrophone to hydrophone. The receiving array is linear and consists of equally spaced hydrophones. The performance of conventional and likelihood‐ratio detectors is analyzed. In an interference‐dominated environment, the likelihood‐ratio detector can be superior by a substantial margin. Complete elimination of the interference can be accomplished at a cost of no more than one hydrophone.