Amicarbazone is a relatively new photosystem II (PS II) inhibiting herbicide introduced for use in turfgrass for selective Poa annua control.  Poa annua resistance to PSII inhibiting herbicides already exists with known resistance to substituted ureas (diuron) and triazine (atrazine and simazine).  Two key mutations in the psbA gene of Poa annua are known to confer resistance PSII herbicides – Ser₂₆₄ to Gly and Val₂₁₉ to Ile.  Ser₂₆₄ to Gly was previously confirmed by the authors to confer resistance to amicarbazone and triazines.  Our objective was two-fold:  to evaluate the effects of Ser₂₆₄ to Gly mutations on diuron response and to locate Poa annua populations with Val₂₁₉ to Ile mutation in order to evaluate the response of amicarbazone, diuron, and triazines to populations with this mutation.  Approximately 30 Poa annua populations have been screened with psbA genes sequenced.  We have concluded that the Ser₂₆₄ to Gly mutation does not confer resistance to diuron, while it does confer resistance to amicarbazone and triazines.  Secondly, while populations were identified with increased tolerance to diuron and susceptibility to triazines and amicarbazone, we have yet to elucidate a molecular mechanism of resistance as no population sequenced contained the suspected Val₂₁₉ to Ile mutation.