Water solubilized nanoparticles such as CdSe-ZnS core-shell nanocrystals (quantum dots, QDs) have great potential in bioimaging and sensing applications due to their excellent photophysical properties. However, the efficient modification of QDs with complex biomolecules represents a significant challenge. Here, we describe a straightforward arylhydrazone approach for the chemoselective covalent modification of QDs that is compatible with neutral pH and micromolar concentrations of the peptide target. The kinetics of covalent modification can be monitored spectroscopically at 354 nm in the presence of the QD and average peptide/QD ratios from 2:1 to 11:1 were achieved with excellent control over the desired valency. These results suggest that aniline catalyzed hydrazone ligation has the potential to provide a general method for the controlled assembly of a variety of nanoparticle-biomolecule hybrids.