The mechanism of ring constriction of a nematophagous fungus has been studied in vitro. The process of ring constriction in vivo normally requires less than one second. However, precise observations of the mechanism are difficult due to this rapidity, as well as the obstruction of the ring by the nematode. A method of initiating ring constriction without the use of an inert mechanical stimulus is described. With this method it is also possible to retard the constriction of the rings so that accurate observations of the entire process can be made. An explanation of the possible physiological mechanism is also presented.