Population dynamics of yellow-bellied marmots (Marmota flaviventris) were studied at Watch Lake, British Columbia. I attempted to determine how population size was regulated and compared life history tactics of the Watch Lake populations with those in other areas. Ninety-three percent of adult females and eight percent of yearling females had litters. The mean litter size was 6'. 1 ± .38. The size and weight of a female's litter were negatively correlated with her reproductive effort in the previous year. The annual mortality rate of juveniles was 62%; mortality of yearlings and adults was 33%. Most yearling males and a few yearling females emigrated. . The populations were expanding at a rate of increase of approximately .27. The Watch Lake colonies were generally larger and denser than those reported from other areas. Adult males were territorial throughout the active season. Adult females defended territories during pregnancy and lactation. This is the first report of female territoriality in Marmota flaviventris. Experiments indicated that' (1) adult males caused yearling males to emmigrate and (2) adult females inhibited reproduction of yearling females. I predict that numbers will stabilize through the territorial behaviour of breeding females either reducing the breeding success of females or increasing the emigration rate of yearling females. I describe an experiment to test this hypothesis. Marmot numbers near Watch Lake have been increasing for several decades as new habitat becomes available. Life history tactics of these populations differs from those inhabiting the stable environment of subalpine Colorado. Marmots at Watch Lake suffered greater mortality. They began breeding at a younger age than those in other populations. They allocated relatively more energy to.reproduction, produced larger litters of smaller young, and grew to smaller adult size. These differences are consistent with those predicted by the theory of r- and K- selection for colonizing and stable populations. / Science, Faculty of / Zoology, Department of / Unknown