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Transcriptionally active Balbiani ring (BR) genes in the salivary glands of the dipteran Chironomus tentans were studied by immunoelectron microscopy to establish the distribution of spliceosome components along a specific pre‐messenger ribonucleoprotein (pre‐mRNP) fiber. The BR genes are 35‐40 kb in size with three introns close to the 5′ end and one close to the 3′ end; a very large middle portion lacks introns. As a rule the 5′ introns are spliced concomitant with transcription in the promoter proximal third of the gene, while the 3′ intron is spliced post‐transcriptionally. The BR genes with growing pre‐mRNPs were visualized in situ, while completed and released pre‐mRNPs were isolated from the nucleoplasm and studied unfolded on a grid surface. An anti‐snRNP antibody (Y12) bound mainly to the promoter proximal third of the BR gene (86%) and only to a minor extent to the middle and distal thirds …