Associated blackfly larvae ofSimulium argenteostriatum(Strobl) andProsimulium hirtipes(Fries) were collected in the Säntisthur, a mountain stream in the eastern part of the Swiss Alps. The overall body length, the head capsule length and the width were measured to assign the larvae to three size categories. In addition, some morphometric characters of the labral primary fans were analyzed: the fan areas, the ray numbers and the distances between these fan rays. Gut dissections showed that there were some differences in the size frequency distributions of ingested diatoms between both blackfly species. The overlap of these size frequency distributions varied between 0.39 and 0.77. Only low proportions between 0 and 4.6% of the ingested diatoms were equal in size or larger than the fan ray distances. Therefore it is assumed that the sieving mechanism of the extended fan rays is not very important for particle capture ‐in contrast to previousinvestigations of other larval blackfly associations.