Diatoms belonging to the class Bacillariophyceae were found on the legs of Libellula sp.'larvae. Using a scanning electron microscope, the raphe and the central and polar nodules were observed on the lower valve (hypotheca), whilst the upper valve (epitheca) was seen to possess a pseudo-raphe (Fig. I). These characteristics place the diatoms in the suborder Monoraphideae of the order Pennales. The absence of a raphe on the upper convex valve is probably related to the habit of these diatoms, i.e. limited mobility (Fig. lb). The lower concave valve bearing the ’true’ raphe (Figs, la, 2) is in close contact with the substrate. Minyte processes occur around its periphery and transverse striations run outwards from the raphe (Fig. 2). These diatoms thus belong to the genus Cocconeis. Members of this genus are typically epiphytic — in this case epizoic — and the peripheral pores are associated with their attachment to the substrate. The rounded, 'pill box’ form of the second diatom is typical of the order Centrales (Fig. Id). The valves are centric in structure with no raphe or pseudo-raphe, indicating