Abstract 1. 1. A mutant of Achromobacter fischeri which requires exogenous aldehyde for light emission has been used to study the rate of penetration of a series of aliphatic aldehydes (heptanal to tetradecanal) through the bacterial cell membrane. 2. 2. As the chain length is increased, penetration into the cell is slower, affinity for luciferase increases, and the maximum light intensity is greater. The properties of the isolated enzyme agree with those observed for the activity in the whole cell. 3. 3. No special transport mechanism for the movement of these aldehydes through the bacterial membrane was revealed by the use of a variety of metabolic inhibitors. 4. 4. It is concluded that for this series of aldehydes lipide solubility is of minor importance for penetration and that the major controlling factors are membrane pore size and molecular mass of the aldehyde molecules.