Frontonia leucas is one of the most common ciliates in all my sites, which have a mud layer of dead plants and leaves. The specimens are found exclusively in the uppermost mud layer and collect in the sample containers at the bottom. In some cases I was able to observe mass developments with hundreds of specimens per milliliter.

Frontonia leucas is quite a large ciliate with an average length of 250 µm and is immediately noticeable in the samples. The ciliate is a fast swimmer. The cytoplasm is vacuolated and appears foamy. The specimens are often filled with dark-colored food vacuoles. These are usually phagocytized cyanobacteria, algae or euglenids (e.g. Trachelomonas).

The oral apparatus is located in the anterior quarter and appears quite small compared to the size of the ciliate. It has three adoral membranelles on the right side, with an undulating membrane on the left, as is typical for the genus Frontonia (s. fig. 5 a-c). Also typical for Frontonia is a preoral and a postoral suture (s. figs. 2 a and 4).  The postoral suture extends from the oral apparatus in an almost straight line to the posterior end (s. fig. 2 a).

The contractile vacuole of Frontonia leucas is located in the middle of the body on the dorsal side (s. figs. 1 a and 3 a-b). In order to drain the water from the large ciliate, the contractile vacuole forms many auxiliary canals that spread out in the cytoplasm in a radial pattern (s. fig. 3 b). They reach their maximum length and expansion during the systole. The contractile vacuole has only one excretory pore (s. fig. 3 a).

The macronucleus is located approximately in the middle of the cell. It is elliptical or ovoid and has 2–9 round micronuclei (s. fig. 6). In my population, however, I could not observe any specimen with more than 5 micronuclei.