Further studies of the threshold‐of‐cavitation noise in liquid helium have been made with an ultrasonic plane‐wave system. The behavior of this threshold was found to have a similarity to that of the specific heat in the λ region. Both above and below the λ temperature, the threshold was very insensitive to variations of static pressure and unchanged by addition of solid air or the imposition of electric fields. These results are difficult to explain on the basis of conventional theories of nucleation, but possibly indicate that nucleation involves the aggregation of submicroscopic structures.