Scanning tunneling microscopy has been used to study the formation of a Bi monolayer deposited on the fivefold surface of i-Al70Pd21Mn9. Upon deposition of low submonolayer coverages, the nucleation of pentagonal clusters of Bi adatoms of edge length 4.9 Å is observed. The clusters have a common orientation leading to a film with fivefold symmetry. By inspection of images where both the underlying surface and the Bi atoms are resolved, the pentagonal clusters are proposed to nucleate on pseudo-Mackay clusters truncated such that a Mn atom lies centrally in the surface plane. The density of these sites is sufficient to form a quasiperiodic framework, and subsequent adsorption of Bi atoms ultimately leads to the formation of a quasicrystalline monolayer. The suggested nucleation site is different from that proposed on the basis of recent density-functional theory calculations.