Exposure of a facultative air breather, Hoplosternum littorale, to 12.5, 25, and 37.5% of the water soluble fraction (WSF) of Urucu crude oil, resulted in a rapid increase in air-breathing frequency (ABF) sustained over the 45 min period of exposure. Following 4 h exposure to a graded increase in WSF up to 50%, there was no significant affect on haematocrit, or plasma [Na+] and [K+]. Crude oil ingestion resulted in some degree of ion regulatory impairment, however results were variable. A single oral dose of 3.0 ml/kg of Urucu crude elevated net whole body Na+ efflux and resulted in a 7% reduction in plasma [Na+] 72 h following ingestion. A single oral dose of 3.0 ml/kg resulted in a significant net whole body K+ efflux and a reduction in plasma [K+] 24 h after ingestion. No mortalities were observed in any exposure regime in this study. An oral dose of Urucu crude oil at 3.0 ml/kg also resulted in a 24% reduction in ATP:Hb ratio (from 0.206 to 0.157) and a 31% reduction in GTP:Hb ratio (0.455 to 0.315) 24 h following ingestion indicating that these fish may be hypoxemic. Taken together, these results indicate that exposure of H. littorale to Urucu crude oil affects gas exchange and ion regulation.