Olfactory receptor neurons select a single odourant receptor gene for expression out of a large gene family. The mechanisms of this extreme selectivity are largely unknown. We have determined in detail the developmental expression dynamics of a representative subset of the zebrafish odourant receptor repertoire, using in situ hybridization analysis. We have thus generated a dataset, which allows us to test hypotheses of odourant receptor gene regulation. The receptors chosen belong to four different groups with respect to ontogenetic onset of expression (onset groups). Statistical analysis of the data supports a model in which the final choice of an individual odourant receptor gene occurs stochastically from within a group of genes sharing a deterministically defined onset of expression. Genomic mapping revealed a pronounced correlation of onset of expression with genomic neighbourhood. During a protracted juvenile developmental period individual regulatory influences seem to modify the expression of odourant receptor genes, a notable example being a transient decrease in expressivity of two odourant receptor genes.