Small calcareous opercula occurring in early Rupelian euryhaline deposits in Limburg (Belgium) have been described by Bosquet (1859, 1861), Sandberger (1861) and Karnekamp (1983). They were interpreted as opercula of the gastropods Stenothyroides pupa (Nyst, 1836) (Truncatelloidea: Stenothyridae) or Sandbergeria cancellata (Nyst, 1836) (Cerithioidea: Diastomatidae), disregarding the fact that neither shape nor morphological details of opercula of living relatives fit either of the two gastropods, which, moreover, are expected to have had non-calcareous opercula which would not be preserved as fossils. These microfossils fit well into the range of opercular morphologies described from living species of the genus Spirobranchus Blainville, 1818 (family Serpulidae, phylum Annelida). They are redescribed as Spirobranchus limburgicus n. sp