Abstract: To determine whether homology-dependent gene silencing or cosuppression mechanisms underlie copy number control (CNC) of Ty1 retrotransposition, we introduced an active Ty1 element into a naive strain. Single Ty1 element retrotransposition was elevated in a Ty1-less background, but decreased dramatically when additional elements were present. Transcription from the suppressing Ty1 elements enhanced CNC but translation or reverse transcription was not required. Ty1 CNC occurred with a transcriptionally active Ty2 element, but not with Ty3 or Ty5 elements. CNC also occurred when the suppressing Ty1 elements were transcriptionally silenced, fused to the constitutive PGK1 promoter, or contained a minimal segment of mostly TYA1-gag sequence. Ty1 transcription of a multicopy element expressed from the GAL1 promoter abolished CNC, even when the suppressing element was defective for transposition. Although Ty1 RNA and TyA1-gag protein levels increased with the copy number of expressible elements, a given element's transcript level varied less than twofold regardless of whether the suppressing elements were transcriptionally active or repressed. Furthermore, a decrease in the synthesis of Ty1 cDNA is strongly associated with Ty1 CNC. Together our results suggest that Ty1 cosuppression can occur post-transcriptionally, either prior to or during reverse transcription.