Conditioned fear in response to explicit and contextual cues was examined using the startle reflex in three groups of participants over two sessions separated by 4-5 days. The conditioned stimulus (CS) was paired with an aversive unconditioned stimulus (US) (shock) during conditioning in the paired but not in the unpaired group. In the reaction time (RT) group, the US was a nonaversive visual signal for an RT task. In the paired group, the CS potentiated startle in the postconditioning phase. This conditioned response was fully retained over the retention interval. There was no substantial change in baseline startle (startle delivered in the absence of CS). By contrast, startle was not potentiated by the CS in the unpaired group, but baseline startle was increased from Session 1 to Session 2. In the RT group, startle was not affected by the CS, and baseline startle was reduced from Session 1 to Session 2. These results suggest that paired presentations of a CS and an aversive US result in conditioned fear in response to the CS but little contextual fear, whereas unpaired presentations of a CS and US leads to poor explicit cue conditioning but substantial contextual fear.