The nature of forgetting in a prospective-memory (PM) task was examined through the retrieval-practice paradigm. In two experiments, participants studied a series of category–exemplar pairs that belonged to one of eight categories (e.g., FRUIT-apple) and then engaged in retrieval practice for three members in each of four categories. At the final test phase, every participant was required to detect target items that belonged to one practiced and one unpracticed category while performing an ongoing task. Results showed the worst detection performance in response to the unpracticed items in the practiced categories versus that in response to the unpracticed items in the unpracticed categories. This retrieval-induced forgetting was smaller for the detection of focal targets, for which the processing largely overlapped with the processing of the ongoing task (Experiment 2), than for non-focal targets, for which the processing was performed independently from that of the ongoing task (Experiment 1).