Mycoplasma hyopneumoniae (M .hyopneumoniae) and Mycoplasma hyorhinis (M .hyorhinis) infections are common in China .To investigate the prevalence of M .hyorhinis and M .hyopneumoniae in Jiangsu Province of China ,a mo-lecular epidemiological survey was conducted from 399 nasal swab samples of unvaccinated pigs using nested Polymerase Chain Reaction (nested PCR) .Nasal swab samples were collected from Jiangquhai porcine lean (JQHPL) strain pigs and other West-ern breeds .Clinical samples were taken from each pig and divided into different groups based on ages of pigs (7 ,14 ,21 ,28 , and 35 days) .Results indicated that the prevalence of M .hyorhinis was 70 .9% from different herds in Jiangsu Province in China ,while the prevalence of M .hyopneumoniae was 13 .5% .M .hyorhinis infection was more common in pigs for less than 5 weeks of age compared to M .hyopneumoniae infection .Co-infection was also observed in 30 samples (7 .5% ) in which both M .hyorhinis and M .hyopneumoniae were detected .The M .hyorhinis infection increased as the animals grew from 7 to 35 days .The M .hyopneumoniae infection did not change significantly as the pigs grew older .Significant difference of M .hyorhi-nis infection was observed between other Western breeds and JQHPL pigs (P<0 .001) .JQHPL pigs appear to be more sensi-tive to the M .hyorhinis infection as compared to the other Western breeds .However ,there is no obvious relationship between the breed type and M .hyopneumoniae infection (P>0 .05) .