Occasionally, consumers complain of insects found in draught beer. Changes in activities of insect enzymes when insect was soaked in beer were investigated in 1994. The cholinesterase activity of the insect decreased gradually, and the length of time the insects were soaked in beer could be estimated from the remained cholinesterase activity. It was possible to determine whether the insect entered the beer during the packaging process or after consumer opened the package. This investigation was carried out on two kinds of insects (German cockroach and house fly). This time changes in cholinesterase and catalase activities of four kinds of insects (American cockroach, Flesh fly, Brown house mosquito, Bumble bee) were investigated to increase application examples. These insects were soaked in two kinds of beer of different pH, and changes in cholinesterase and catalase activities were investigated. Beer pH did not influenced on the change of the cholinesterase activity of the insect, and high correlation between cholinesterase activity and the length of time the insects were soaked in beer was confirmed. Cholinesterase activity even of a very little insect like Brown house mosquito could be measured, and it confirmed that it was an effective method to estimate the time point of insect entered in draught beer. As for the catalase activity, the correlation between catalase activity and the length of time the insects were soaked in beers was lower than that of cholinesterase activity, and there was also a case that no correlation was observed.