The presence and spread of clinically important antibiotic-resistant bacteria in reservoirs from natural environments are not well studied compared to the clinical environments. The overall aim of this project was to study the presence of clinically important antibiotic-resistant bacteria in a reservoir from natural environments. Wild birds were chosen not only as indicators of the level of antibiotic resistance in surrounding natural bacterial populations, but also since birds can act as vectors of several potential pathogens including enteropathogens and because they by migration have an ability to spread these pathogens across geographical regions. The studies in this thesis showed that wild birds carry antibiotic-resistant enterobacteriaceae. The levels and spectrum of antibiotic resistance varies between different bird populations and geographical regions. In bird populations without interaction with human activities throughout the year, antibiotic resistance is lacking. Antibiotic-resistant bacteria could however probably be dispersed to remote regions by bird migration. Extended-spectrum beta-lactamases (ESBLs) and especially CTX-M types are found in comparable high levels in gull populations considering the recent emergence of these resistance genes in clinical settings. The CTX-M types found in wild birds are the same types that are found in clinical settings and in food producing animals from the same regions. ESBL-producing E. coli isolated from Yellow-legged Gulls are genetically heterogenous, reflecting that these resistance genes are present across the full E. coli genetic diversity. In wild birds CTX-M are found both in E. coli strains with previously known “human signature” as well as “novel” strains. This indicates that these genes are indeed very mobile and rapidly dispersing both through horizontal gene transfer and through successful clones. The findings in this thesis indicate that bird colonies could act as melting pots and reservoirs for new resistance types and that wild birds could act as important indicators of the level of antibiotic resistance dispersal in natural environments.