This study examined the time course of object naming in 21 individuals with primary progressive aphasia (PPA) (8 agrammatic (PPA-G); 13 logopenic (PPA-L)) and healthy age-matched speakers (n =17) using a semantic interference paradigm with related and unrelated interfering stimuli presented at stimulus onset asynchronies (SOAs) of −1000, −500, −100 and 0ms. Results showed semantic interference (SI) (i.e. significantly slower RTs in related compared to unrelated conditions) for all groups at −500, −100 and 0ms, indicating timely spreading activation to semantic competitors. However, both PPA groups showed a greater magnitude of SI than normal across SOAs. The PPA-L group and six PPA-G participants also evinced SI at −1000ms, suggesting an abnormal time course of semantic interference resolution, and concomitant left hemisphere cortical atrophy in brain regions associated with semantic processing. These subtle semantic mapping impairments in non-semantic variants of PPA may contribute to the anomia of these patients.