Crithidia are related to other trypanosomatid parasites which cause African Sleeping Sickness (T. brucei spp.), Chagas' disease (T. cruzi) and Leishmaniasis (Leishmania spp.) Crithidia fasciculata are cheap to grow without CO2 in inexpensive serum-free media, to densities over 10^8 cells per mL with doubling times of around 4 hours. These properties facilitate easy biochemistry experiments with large parasite numbers.  Their genome is available, though genetic tools can be difficult.