Summary 0[ This paper explores the concept of the critical community size for persistence of infection in wildlife populations[ We use as a case study the 0877 epidemic of phocine distemper virus in the North Sea population of harbour seals\ Phoca vitulina[ 1[ We summarize the available data on this epidemic and use it to parameterize a stochastic compartmental model for an infection spreading through a spatial array of patches coupled by nearest!neighbour mixing\ with replacement of susceptibles occurring as a discrete annual event[ 2[ A combination of analytical and simulation techniques is used to show that the high levels of transmission between di}erent seal subpopulations\ combined with the small annual birth cohort\ act to make persistence of infection impossible in this harbour seal population at realistic population levels[ The well known mechanisms by which metapopulation structures may act to promote persistence can be seen to have an e}ect only at weaker levels of spatial coupling\ and higher levels of host recruitment\ than those empirically observed[ We ask which of the variables of this critical meta! Reviewing that mass mortality\ Hall "0884# com! population distribution are central in determining per! mented that {if PDV behaves like other morbilliviruses sistence] for example\ is it more important to know it should have been eliminated from the North Sea\ patch population sizes\ the number of patches\ the because the surviving population of susceptible ani! level of interpatch mixing or the total population size< mals is too small to allow the disease to persist or to Metapopulation theory has been used to estimate the permit the establishment of a new epidemic|[ This minimum amount of suitable habitat Ð MASH "Han! concept of a threshold susceptible population has been ski\ Moilanen + Gyllenberg 0885# Ð necessary for a a central one in ecological epidemiology since the population to persist[ On the other hand\ conservation work of Bartlett on measles 39 years ago "Bartlett biologists have attempted to estimate the minimum size necessary for a population to have a particular 0845# and much theoretical work has been concerned to establish that persistence properties can be strongly probability of persisting for a certain length of time "the minimum viable population\ MVP "Soule 0876dependent on mixing structure[ In this paper we use the PDV example to study how the mixing that arises Bjo Ârge\ Steen + Stenseth 0883##[ The fact that the theoretical relationship between patch size and local from a patchy host population a}ects persistence[ A common theoretical approach to this problem is to extinction is well understood in an epidemiological setting has enabled us to investigate the way in which interpret the host patches as habitat patches of a pathogen metapopulation "Hanski + Gilpin 0880#[ local and total population size interact in determining