The aim of the study was to evaluate the colonizing ability and the invasive capacity of selected Campylobacter jejuni strains of importance for the epidemiology of C. jejuni in Danish broiler chickens. Four C. jejuni strains were selected for experimental colonization studies in day-old and 14-day-old chickens hatched from specific pathogen free (SPF) eggs. Of the four C. jejuni strains tested, three were Penner heat-stable serotype 2, flaA type 1/1, the most common type found among broilers and human cases in Denmark. The fourth strain was Penner heat-stable serotype 19, which has been shown to be associated with the Guillain Barré Syndrome (GBS) in humans. The minimum dose for establishing colonization in the day-old chickens was approximately 2 cfu, whereas two- to threefold higher doses were required for establishing colonization in the 14-day-old chickens. Two of the C. jejuni strains were shown to be invasive in orally challenged chickens as well as in three different human epithelial cell lines.