The occurrence of some bacteriform Microsporidia from certain Crustacea and Insecta in Canada and in England is recorded.

The numbers of hosts examined and those that were infected with Pyrotheca incurvata, Octosporea muscae damesticae, Toxoglugea, Spiroglugea and Telomyxa spp. innom., are tabulated.

Descriptions of the bacteriform Microsporidia are given and illustrated. Differential diagnoses are given as far as possible.

The effects of the parasites on the hosts and their distribution within the hosts are set forth.

The lethal potentialities of the Microsporidia are indicated both in natural infections and by experimental ones.

The economic importance and possible applications of these bacteriform Microsporidia are considered.