Oxygen gradients can rapidly develop in fluid films covering biofilm in trickling filters. We use a computer model to calculate the maximum oxygen flux to a biofilm, and show that the maximum ammonia removal rate in nitrifying trickling filters (NTFs) is within the range expected from oxygen-limited transport. Based on our experience with NTFs, we discuss the implications of gas-transport limitations for methanotrophic biofilms in plastic media trickling filters.