A study was conducted on the efficiency of phycocyanin extraction from Spirulina platensis (Arthrospira platensis) cells disrupted by ultrasonic irradiation. Extraction followed first-order kinetics with respect to the length of time for irradiation. The first-order rate constant increased linearly with the output of ultrasonic irradiation. In order to extract phycocyanin there was an appropriate range of ultrasonic frequency, f(u). In addition the most important finding is that the purity of phycocyanin in its crude extract depended on f(u). For example, phycocyanin was extracted with higher purity at f(u)=28 kHz than at f(u)=20 kHz. It is suggested that rapid and selective extraction of phycocyanin from S. platensis may be possible if an optimized ultrasonic application is developed for a given suspension.