The reliability of the estimation of residency time of the bank vole Clethrionom ys glareolus (Schreber, 1780) and the yellow-necked mouse Apodem us flavicollis (Melchior, 1834) in open populations was assessed by halving the 6-w eek intervals between trapping sessions. The status of " single-capture" individuals was identified to estimate the probability that they are native young rather than adult invaders. W e found that the majority of " single-session" individuals constituted true transients that were present in the study plot for a short time only. They were young, immature rodents with a small body mass and a small number o f captures. The probability that the " single-session" individuals stay on the plot for a longer time was 0 .35 and 0.37 for voles and mice, respectively. These ostensible transients were trap-prone, fully-grown, mature adults, that revealed their presence on the plot already at the beginning of each trapping session. W e found that " single-capture" individuals were mainly young, immature rodents that were not retrapped. The probability that the single-capture rodents are mature individuals, with a longer residency tim e, was 0 .10 and 0 .18 for voles and mice, respectively.