Fourteen species of ciliates, seven of which are new, were found livin
g in a sample of anoxic water collected from a small lake in Spain. Th
e species belong to all six orders in which anaerobic ciliates have be
en described and they include the first anaerobic representatives of t
he order Prostomatida. This surprising diversity is probably sustained
because it embraces all ciliate feeding types, and because protozoa a
re the only important consumers of the diversity of microbes in anoxic
habitats. Six of the anaerobic ciliate species have aerobic congeners
; this strengthens the contention that anaerobic ciliates evolved inde
pendently from aerobes belonging to several taxonomic groups.