Experimental studies of fault-finding performance typically take place over a limited number of sessions. This paper reports a study of extended practice of fault location in networks of logic units. Perfect performance was never achieved by subjects participating in the experiment. Error rates on somemeasures ofdiagnostic efficiency increased over the course of the experiment. Diagnostic efficiency was also affected both by the perceptual cornpIexity of the format representing the system and by the complexity of the system itself (presence or absence of OR gates). Errors in the diagnosis of faults can be attributed both to diagnosis with inadequate information and to lapses in low-level operational skills (typing errors).