Technical Abstract: The use of antimicrobials in food animal production, particularly those commonly used to treat infections in humans, has become a source of debate in recent years. However, limited data are available regarding the development of resistance following subtherapeutic or therapeutic administration of antimicrobials in animal production. The objective of this study was to evaluate the effect of administration of therapeutic and subtherapeutic levels of tylosin on erythromycin susceptibility of C. jejuni and C. coli isolated from ceca of treated broilers. In three replicate studies, day-of-hatch chicks were exposed to macrolide susceptible C. jejuni or C. coli. At two weeks of age, tylosin was administered at subtherapeutic (22 ppm, continuously in the diet) or therapeutic levels (529 ppm in drinking water for 5 days). Broilers were sacrificed weekly. Total and erythromycin resistant Campylobacter were enumerated from individual ceca with contents. Overall erythromycin resistance was observed at a higher frequency (P<0.01) among C. coli isolates 70.8% compared to C. jejuni isolates 36.8% following tylosin administration. Across Campylobacter species erythromycin resistance was observed at a higher frequency (P<0.001) when tylosin was administered at subtherapeutic (62.7%), compared to therapeutic levels (11.4%). Subtherapeutic administration resulted in recovery of 83.3% and 56.1% erythromycin resistant isolates compared to only 33.3% and 7.9% of the isolates expressing erythromycin resistance following administration of therapeutic levels, for C. coli and C. jejuni, respectively. Further studies are needed to determine the factors involved in the apparent difference in the acquisition of macrolide resistance in C. coli compared to C. jejuni.