International audience ; Before the year 2000, Mycoplasma synoviae was mainly associated with subclinical respiratory infections in broilers and was considered to have low clinical and economical impact in the Netherlands. Due to the occurrence of M. synoviae arthritis and amyloid arthropathy, and eggshell apex abnormalities added to the increasing demand for M. synoviae free poultry this has changed. Therefore, a cross-sectional seroprevalence study was carried out in a twelve month period during 2005 and 2006. Ten blood samples per farm were used as M. synoviae was expected to spread quickly except for grandparent and layer breeder stock where 24-60 blood samples per house derived from a voluntary M. synoviae monitoring programme were used. Sera were tested by means of Rapid Plate Agglutination (RPA) test (agglutination at dilution "1:8 was considered positive). The farms sampled out of the national total were: broiler grandparent 53/53, broiler parent rearing 34/150, broiler parent 114/300, broiler 185/800, layer grandparent 13/13, layer parent 40/50, layer 173/1250 and meat turkey 50/75. The seroprevalence of M. synoviae in commercial poultry was high, especially in commercial layers where it was 73% (95% C.I.(67-80)); in layer and broiler grandparent stock it was 0.0% and 11% respectively, based on sample sizes equal to the population size. In layer and broiler parent farms it was 25% (95% (C.I. (19-31) and 35% (95% C.I. (28-44)), respectively; while in broiler parent rearing and broiler farms it was both 6% (95% C.I. (0-13) and (3-9), respectively); in meat turkey it was 16% (95% C.I. (10-22)).