According to dynamic pattern theory, intrinsically stable bimanual coordination patterns affect, and are affected by, the acquisition of a new coordination pattern. In Experiment 1, subjects practiced either a 45 degrees or a 135 degrees relative phase pattern for 4 days; in Experiment 2, they practiced a 90 degrees relative phase pattern over 6 days. Retention tests were conducted 4 weeks after the last practice session in both experiments. Performance on both the in-phase (0 degree) and anti-phase (180 degrees) patterns was also measured on each day. Contrary to predictions, the experiments revealed that reciprocal effects between the intrinsic patterns and the new pattern were only temporary, and did not affect learning in any permanent way. As well, learning a new pattern was not differentially affected by its relation to an intrinsic pattern. Though commonly believed to be prevalent in skill acquisition, negative transfer effects are rather uncommon in motor learning research (Schmidt, 1988). One reason for this is that the locus of negative transfer effects is cognitive, found most commonly in tasks where the translation required between thought and action has been altered from what might be considered natural (e.g., Lewis, McAllister, & Adams, 1951). It is this issue of what is natural that is the core of the present research: how something that can be performed "naturally" (efficiently and without practice) affects new learning and how new learning affects what was once considered natural. Most would agree that the acquisition of a new motor skill is influenced by the existing skills possessed by an individual (Adams, 1987; Schmidt, 1988). For many researchers, however, this influence is little more than an annoyance, and motor learning research is replete with laboratory tasks that were created to avoid the influence of existing skills on new learning. In some cases, tasks were designed to be so simple that subjects were assumed to have a mastery of the fundamental skills involved in achieving successful performance before the new task was practiced. Reaction time, line positioning, and target aiming tasks are examples. Another type was the "novel" task, which was designed to require motor skill that no subject would have acquired previously. In theory, a novel task required new learning, and the uniqueness of the task ensured that existing skills would contribute minimally to new learning. Rotary pursuit tracking, mirror tracing, and free-standing ladder climb tasks are examples of novel tasks that have been used for this purpose. However, as with very simple tasks, the use of novel tasks provided little information about the role of existing skills in new learning. When the influence of existing skills on new learning was the purpose, researchers faced a different problem the experimental design. Historically, researchers typically used a transfer design for this purpose (see Schmidt, 1988, for review). For example, one group of subjects would learn Task A followed by practice on Task B. Their performance on the second task (the transfer task) was compared to the performance of a control group that learned Task B without having previously learned Task A. The relative performance of the experimental and control groups on Task B reflected the influence of previous experience on Task A. Although this design has been used extensively (Adams, 1987), there are limitations. The most important is the problem of individual differences, as the influence of previous learning is made by comparing separate groups of individuals. The potential problems with this type of between-group design include Hawthorne effects, counterbalancing, and equating group performances prior to practice (see Schmidt, 1988, Chapter 11, for more discussion). As well, the amount of practice that is given to the experimental group on Task A is a critical variable: how well Task A has been learned, and how this is evaluated (i.e., what performance on task A is compared to). …