Atmospheric ice nucleus and aerosol samples were collected with an aircraft at various altitudes over the ocean upwind of the Hawaiian Islands on four days in June 1980. Below the subtropical trade wind inversion, ice nucleus activity was detectable at -10 ¿C in concentrations of 11-1 active at -12 ¿C. Above the inversion, threshold activity decreased to -15 ¿C and colder. Ice nucleus concentrations were about 10-5 of the number of aerosol particles at all levels.