The peniculids are an order of ciliate protozoa, including the well-known Paramecium and related genera, such as Frontonia, Stokesia, Urocentrum and Lembadion.

Typically the body has uniform, dense cilia, which also cover a vestibule preceding the mouth.

Extrusomes are characteristically in the form of spindle trichocysts, which release thread-like shafts, and never mucocysts.

Nematodesmata (rods) arise from the bases of the oral or perioral cilia, but these do not support a cyrtos as in some other classes.

They were divided into two suborders by Small and Lynn in 1985, who placed them in the class Nassophorea, owing to ultrastructural peculiarities such as the presence of nematodesmata, which was considered to indicate the cyrtos was secondarily absent.