Measurements were made of the cavitation threshold for polypropylene glycol and for castor oil in a cylindrical resonator at approximately 16.5 kHz. The threshold for degassed castor oil was ∼233 dB//iμPa peak, in agreement with the measurements of Briggs, Johnson, and Mason [J. Acoust. Soc. Am.19, 661–664 (1947)]. Degassed polypropylene glycol exhibited wide variability in threshold, with an initial value ∼240 dB//1μPa peak, dropping quickly to values ∼225–228 dB//1μPa peak. Partial restoration to higher values could be attained after a five‐hour rest period. [Work supported by Naval Sea Systems Command.]