This is the accepted version. It'll be replaced with the final pdf when it's available. ; Background Influenza A/H7N9 virus emerged in eastern China February 2013, and continues to circulate but ecology is poorly understood. Methods In March 2013, Guangdong Centers for Disease Control (CDC) implemented environmental- and human syndromic surveillance. Environmental samples from poultry markets in 21 city CDC?s (n= 8,942), and respiratory samples from individuals with influenza like illness or pneumonia (n= 32,342) were tested by RT-PCR. Viruses isolated from 16 patients and 6 environmental samples were sequenced. Results A steep increase in human cases was observed starting November 2013, coinciding with emergence of influenza A/H7 virus in environmental surveillance. Sequence analysis showed co-circulation of 4 reassortant viruses that evolved from the original virus by reassortment with local avian influenza A/H9N2 viruses. Conclusion: The steep increase in notifications of human cases in Guangdong late 2013 was caused by new, locally acquired reassortant viruses, reflecting widespread undetected local transmission. ; This study was financially supported by 12th five-year-major-projects of China?s Ministry of Public Health. Grant No: 2012zx10004-213 and by the PREDICT Surveillance Animal Human Interface Project of GVF. Grant No: Gvf: 06-09-057-02.