Abstract
In 1961 I reported the first successful attempt to cryopreserve and store protozoa in liquid nitrogen. In this presentation, drawing on more than 30 years of personal experience, I discuss the factors which I consider to be most critical to the successful preservation and long term storage of several species of parasitic protozoa in liquid nitrogen. I then present my most successful protocol for cryopreservation of these parasites. Finally. I cite some longevity records for a variety of protozoa stored in liquid nitrogen vapor at the Laboratory of Parasitic Diseases. The oldest record is for a monoxenic culture of Entamoeha histolytica grown in association with Crithidia fasciculata that was frozen on January 6, 1961. Thirty‐two years and 11 months later both parasites were recovered and used to initiate new culture lines.