It is commonly stated that natural gas-fueled forklifts produce less emissions than propane-fueled forklifts. However, there is relatively little proof. This paper reports on a detailed comparative study at one plant in Edmonton, Canada where a fleet of forklift trucks is used for indoor material movement. (For convenience, the acronym NGV, ie. Natural Gas Vehicle is used to designate natural gas-fueled and LPG, ie. Liquified Petroleum Gas, is used to designate propane-fueled forklifts).Until recently the forklift trucks (of various ages) were LPG carburetted units with two-way catalytic converters. Prompted partially by worker health concerns, the forklifts were converted to fuel injected, closed-loop controlled NGV systems with three-way catalytic converters. The NGV-converted forklifts reduced emissions by 77% (NOX) and 76% (CO) when compared to just-tuned LPG forklifts. After a period of operation, LPG forklifts drifted out of tune while the NGV-converted forklifts maintained similarly low emission levels. In this condition, the NGV-converted forklifts produced lower emissions by 97% (CO) and 84% (NOX). Forklifts with older LPG fuel systems tended to drift out of tune faster and produced much higher emissions than the newest ones. With conversion to new NGV fuel injection systems, the emissions of both old and new forklifts were reduced to essentially the same low levels. Tests also simulated maintenance failures on the NGV fuel systems. In event of a complete catalyst failure, the NGV systems provided substantially lower emissions than the LPG systems because their engine-out emissions were lower, (57% less CO and 27% less NOX). Also, the Three-way catalysts had a much higher effectiveness with the NGV fuel system than when installed on the carbureted propane systems. An exhaust gas oxygen sensor failure put the NGV fuel systems into open-loop mode but their emissions were still lower than normal operation on LPG, (92% less CO and 73% less NOX). An engine coolant temperature sensor failure only slightly degraded the emissions of the NGV fuel systems.