The protozoa, or 'first animals', are a diverse group of unicellular, animal-like organisms. Many protozoa are parasites of man, causing such diseases as malaria and sleeping sickness, which kill millions \Jf people each year. There are also many types of protozoa that are free-living in the natural aquatic environment. It is the purpose of this booklet to introduce the reader to the diversity of these free-living forms and to some basic practical aspects of their biology and ecology.