Although dramatic millipede outbreaks have been reported worldwide, there is no data available on the intrinsic rate of increase (r_m) of these arthropods. This parameter was estimated in the west-European millipede Polydesmus angustus Latzel, a seasonally-breeding species with mixed 1-yr and 2-yr life cycles within populations. Individuals from 20 broods were reared throughout their life cycle under seasonal conditions simulated in the laboratory (monthly mean temperatures and naturally varying photoperiods), to determine age-specific survival and fertility in annual (1-yr life cycle) and biennial (2-yr life cycle) females. The finite rate of increase (λ_m) was calculated using a periodic matrix model and r_m was found to be 5.85 per year. This estimate suggests that outbreaks of annual polydesmidan species in greenhouses or under favorable field conditions can be generated by a small number of fertilized females in 2 yr. The relevance of seasonal conditions to measure r_m per year in long-lived arthropod species that breed seasonally is emphasized. Estimates such as that obtained in P. angustus integrate most seasonal adaptations of the species and are ecologically plausible in the absence of extrinsic sources of mortality. Finally, the results show that biennial females represent a very small proportion of individuals in the stable population structure, but analysis of short-term dynamics indicates that they may be more successful as colonists than annual females.