Abstract We study the microstructure of the Pink Sheets and assess the ability of existing theory to capture salient features of this relatively unstructured and unregulated market. Clustering patterns in quotes, quoted spreads, and trade prices indicate that market participants have selected price-dependent tick sizes for different stocks. Clustering intensity varies across stocks as a function of proxies for information availability. Similarly, the bid-ask spread varies as a function of volatility and liquidity. These results suggest (1) microstructure research has established robust predictions of market attributes and (2) unstructured markets are able to develop at least some effective behavioral norms endogenously.