Many glandular trichomes are key sites for the biosynthesis or secretion of secondary metabolites. In view of this, an understanding of the regulation of their development is of special interest. Nicotiana benthamiana, the plant chosen for this work, possesses two different types of glandular trichomes: one with a single-cell stalk and a four-cell head (short trichomes) and one with a four-cell stalk and a single-cell head (long trichomes). From experiments of Glover et al. (1998) and Payne et al. (1999), it appears that an R2R3 transcription factor is involved in the initiation of biogenesis of glandular trichomes. Using degenerate primers annealing to conserved MYB motifs, fragments representing six different MYB genes were amplified from N. benthamiana cDNA using PCR. For one of these genes (nbmybtri5), it was possible to demonstrate a role in the development of long trichomes. The silencing of this gene via VIGS (virus induced gene silencing) and a stable transformation with Agrobacterium tumefaciens resulted in a significant reduction in the total number of trichomes and specifically in the number of long trichomes. On the one hand, NbMYYBTRI5 has a high homology to AmMYBML1 from Antirrhinum majus, for which a function in the development of glandular trichomes on a defined part of ventral petals is known. On the other hand, NbMYYBTRI5 has a low homology to those MYB transcription factors playing a role in initiation of development of simple trichomes, such as GL1 of Arabidopsis thaliana. In contrast to these proteins, NbMYYBTRI5 does not contain the amino acids responsible for an interaction with bHLH proteins. This corroborates the hypothesis that other regulatory proteins are involved in the development of glandular trichomes than those responsible for the development of simple trichomes.