Aims: To compare techniques for assessing biofilm populations previously subjected, or not, to inimical treatment by chlorine or drying. Methods and Results: Four sonication treatments and swabbing were compared on Salmonella Typhimurium or Pseudomonas fluorescens biofilms grown on polyvinyl chloride (PVC). The apparatuses emitting the highest ultrasound energy were the most efficient except on Salm. Typhimurium biofilms subjected to drying: a lethal effect, leading to an underestimation of at least 1.5 log (CFU cm(-2)) was observed with the more aggressive treatment. The differences between the highest count and the swabbing counts ranged from 0.3 log (CFU cm(-2)) (untreated Salm. Typhimurium) to 1.7 log (CFU cm(-2)) (chlorine treated Ps. fluorescens). Impedance measurements, used to assess populations without detaching any cells showed that the calibration curves that were built up from data obtained with suspended cells plus PVC slides were not appropriate. Conclusions: High energy ultrasound techniques designed either for in vitro or in situ studies proved efficient in assessing the number of attached CFU. However, the right treatment duration has to be carefully established before using high energy ultrasound techniques. Significance and Impact of the Study: The effectiveness of a cleaning and disinfection regime inferred from data obtained after swabbing may be greatly underestimated.