Acquisition of the classically conditioned eyeblink response is generally regarded as one of the most basic forms of associative learning. A great deal is known about how the brain encodes this simple form of learning, so that performance of this task may be an indirect indicator of brain functioning. Individual differences in response acquisition have been revealed, but largely ignored, in the research literature. We tested the temporal stability and familial origins of these individual differences using a classic twin study design. Results reveal substantial individual differences in acquisition of the conditioned eyeblink response. These differences are stable across brief retest, and differences in response acquisition exhibit familial aggregation, apparently due, in part, to genetic resemblance.