The specimens of Colpidium kleini shown below come from a culture that Holger Weis kindly made available to me. I myself have so far been unable to detect Colpidium kleini at my local sampling sites, although the species is described as very common.

Colpidium kleini is characterized by a slender, somewhat kidney-shaped body. The anterior third is slightly slendered and inclined somewhat ventrally. The specimens were all between 70–75 µm long. The mouth opening is located in the anterior third. At the posterior end there are some elongated caudal cilia (s. fig. 1 c).

An important distinguishing feature from the similar species Colpidium colpoda is the number of longitudinal rows of cilia. In a slightly compressed specimen, I was able to count 13 rows (s. fig. 4). Calculated over the entire circumference, there are about 30–36 rows. This makes it possible to rule out Colpidium colpoda (50–63 rows). For Colpidium kleini 32–44 longitidinal rows of cilia are specified what fit my observations. The second similar species Dexiostoma campylum can also be ruled out by the position of the contractile vacuole (in posterior third) and the shape of the macronucleus (spherical). In my specimens the contractile vacuole was located shortly below the cell equator (s. fig. 1 c) und in all specimens the macronucleus was ellipsoid (s. fig. 5). I attribute the small size of my specimens (70–75 µm) on the culture conditions. The common size for Colpidium kleini is about 100 µm.