Paramecium aurelia is the subject of a theory of behavior composed of postulates describing the presence, action, and interaction of three pacemakers: a posterior pacemaker which produces ciliary beat backward along the kineties; a buccal pacemaker which produces ciliary beat toward the suture line and the buccal overture in the vestibulum ; and an anterior pacemaker which produces ciliary beat away from the suture line and the buccal overture in the vestibulum and forward along the kineties elsewhere. Commonly observed behaviors of paramecia are derived from the theory and six experiments relevant to the theory are described. In three experiments, the culture medium in which paramecia were observed was varied. Presence of bacteria and non-nutritive particulate matter decreased swimming scores and increased swimming turn rates. In one of these experiments prior as well as immediate presence of bacteria in culture fluid varied. Prior availability of food bacteria increased swimming scores and contact turn rates. These results are consistent with the theory if it is assumed that immediate presence of particulate matter increases buccal pacemaker activity and that prior availability of food bacteria increases posterior pacemaker activity. In another experiment animals were observed during and following binary fission. During fission swimming scores decreased, and following fission the proters had lower swimming scores and contact turn rates than opisthes. These results are consistent with the theory if it is assumed that periods in which all ciliary beat is absent occur in parent animals during fission, and in proters following fission. Such periods were observed in dividing animals. Opisthes had higher swimming turn rates following fission than prior to it; this result is consistent with the theory if it is assumed that newly organized buccal pacemakers are unusually active. In two other experiments, the anterior tips, buccal cavities, and posterior tips of paramecia were subjected to ultraviolet microbeam irradiation. Irradiated animals moved more during and immediately following irradiation than they moved prior to irradiation or than control animals did. Posteriorly irradiated animals moved forward, anteriorly irradiated animals moved backward. Irradiation also influenced behavior during a subsequent two-minute observation period. Anteriorly irradiated animals had higher swimming scores, lower contact turn rates, and a greater tendency to swim backward. Animals irradiated in the buccal cavity had lower swimming turn rates and higher swimming turn durations; posteriorly irradiated animals had lower swimming scores, contact turn rates, and swimming turn rates. These results are consistent with the theory if pacemakers near irradiated areas are temporarily activated by irradiation and are hyposensitive and hyperreactive following irradiation. A number of research problems were suggested.