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1. Bionomical and ecological investigations on the yellownecked fieldmouse were carried out in forest habitats of four phytosociologically different areas in the Southeast of Lower Austria and in northern Burgenland between 1963 and 1966. A total of 418 individuals of A. flavicollis were marked and registered by 1226 trappings. 2. In two species, A. flavicollis and A. sylvaticus, winter breeding was observed in a young coniferous forest in the Gfieder area in 1963/64. From the population dynamics it can be concluded that winter breeding had also occurred in 1962/63. 3. Comparing the different areas in different years no regularity was found in the start and end of reproduction period. 4. Maximum weights of males of A. flavicollis were different in the four areas (see Table 5). The following percentages of weights over 35 g were found: 5% (Gfieder), 13% (Strelzhof), 7% (Hernstein) and 28% (Neusiedler Wald). 5. During an investigation period from spring to autumn of the next year an average life time of 3,57 (Gfieder), 3,94 (Strelzhof) and 3,53 (Hernstein) months was determined for A. flavicollis. 6. In all areas the yellow-necked fieldmouse was a typical forest species prefering dense and covered habitats, while the long-tailed fieldmouse, which has a broader ecological spectrum, showed different population densities in the course of a year which can only be explained by a change of habitat. 7. Due to the constantly high infestation with larvae of Ixodes ricinus, the yellow-necked fieldmouse can be regarded as one of the most important hosts of this tick species.