ALTHOUGH one of the earliest events of fertilisation of the sea urchin egg is a depolarisation and decrease in resistance of the egg plasma membrane1-3, it is not known exactly what change occurs immediately after the interaction with a single spermatozoon. We report here that in Paracentrotus lividus this change is a depolarisation step of 1-2 mV, occurring 13 s before the main electrical change. The step is accompanied by an increase in cell noise and a decrease in resistance. In other biological membranes, such an observation has been interpreted as the introduction of ionic channels into the membrane4,5. From this analogy we calculate that the single channels introduced into the egg plasma membrane during fertilisation have a conductance greater than 33 pS.