Presented serial patterns of 6 lights to a total of 151 undergraduates in 5 experiments. Ss attempted to track the lights by pressing the appropriate button as the light came on, at a rate of 1 light per 7 sec. The main indicator of difficulty was an error or omission immediately after a correct response (a C-E transition) indicating that S got off the track at that point. Data show that (a) the beginning and sometimes the end of a subunit (run, trill, or repetition) were difficult; (b) organized arrangement of subunits led to far better performance than disarrangement of the subunits; (c) tendencies to use runs vs. trills at 1 point in a sequence depended upon the frequency of such subunits elsewhere in the sequence; (d) error patterns faithfully mirrored a hierarchical organization built into the sequence; and (e) if a single element of a sequence deviated from a hierarchical tree, errors were concentrated in the branch of the tree affected. Results span serial pattern learning, perception of sequential patterns, and coordinated motor skill. (PsycINFO Database Record (c) 2006 APA, all rights reserved)