Extensive reading has been drawing increasing attention from L2 educators as a potentially effective form of L2 reading instruction. However, some researchers argue that it is not easy to observe the benefits of extensive reading in the short term. The present study was motivated by the need to elucidate the differential effects of extensive reading on different aspects of foreign/second language ability, some of which may improve more quickly than others. Development of general reading ability and lower-level linguistic ability was examined. Improvement from a pretest to a posttest was found to be significant for reading ability, but not for linguistic ability. The results suggest that the effects of extensive reading might be manifested more quickly in general reading skills than in L2 linguistic ability, at least for adult L2 learners. Possible reasons for this difference in the progress of different aspects of L2 ability are discussed. (Contains 1 table.)