The respiratory rate of Phragmites australis exposed to synthetic sewage water (SSW) and chlorothalonil (CLT) was evaluated using a respirometer. The study was divided into two phases with the following treatments: phase I with a) tap water, b) tap water + P. australis, c) ARS; phase II with a) SWW+ CLT (0,29 mg L-1), b) SWW + CLT (2,18 mg L-1), c) CLT (0,29 mg L-1), d) CLT (2,18 mg L-1). Each experiment was done by replicate. The influence of relative humidity (RH), solar radiation (SR), air temperature (T), barometric pressure (BP), evaporation and potential evapotranspiration (ETo) on P. australis respiration rate was analyzed. No significant differences were found on the respiratory rate of P. australis (p < 0.05) using tap water and SWW during phase I experiments; the same finding was seen for all phase II experiments. Meteorological variables affected the respiratory rate in all treatments. ETo, T and SR showed significant positive correlation with respiratory rate; RH and BP, a negative correlation.