The lateral flagellum of the antennule of the spiny lobster Panulirus argus houses more than 1,000 morphologically similar olfactory sensilla, called aesthetascs. By using a high-resolution activity labeling technique that depends on entry of agmatine into olfactory receptor neurons (ORNs) through cation channels during odor stimulation, we examined the distribution of different functional types of ORNs within and across mature aesthetascs. A significant number of ORNs in mature aesthetascs are labeled with agmatine during stimulation by single odorants, including adenosine-5'-monophosphate, ammonium chloride, cysteine, glycine, proline, and taurine. The percentage of ORNs per aesthetasc that was agmatine labeled during odor stimulation averaged 0.5-1.6% for single compounds and 4.6% for a 33-component mimic of oyster tissue. For most antennules and antennular regions studied, the percentage of agmatine-labeled ORNs by stimulation with single or complex odorants was statistically homogeneous across most or all aesthetascs. The extent of heterogeneity among mature aesthetascs was correlated with their age: extensive heterogeneity was observed only in the distal part of the flagellum containing the oldest aesthetascs and their ORNs. Thus, it appears that over most of the length of the aesthetasc-bearing region of the lateral flagellum, different and distinct functional types of aesthetascs do not exist. Rather, aesthetascs appear to be repetitive morphological and functional units in olfactory coding. However, because odor sensitivity of ORNs can change with the age of an aesthetasc, some development-related functional heterogeneity exists among aesthetascs.