Following a request from the European Commission, the EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP) was asked to deliver a scientific opinion on the safety and efficacy of a product containing β-mannanase produced by a non-genetically modified strain of Aspergillus niger (CBS 120604). The commercial name is Nutrixtend Optim and it is intended to be used as a zootechnical feed additive for all poultry for fattening. Based on a tolerance trial in chickens for fattening and the no observed adverse effect level identified in a subchronic oral toxicity study in rats, the additive was considered safe for all poultry for fattening. The Panel concluded that the use of the product as a feed additive does not give rise to concerns for consumers and the environment. The additive is considered an irritant to skin and eyes and a dermal sensitiser. Due to the proteinaceous nature of the active substance, it is also considered a respiratory sensitiser. The Panel concludes that the additive has the potential to be efficacious as a zootechnical additive at the level of inclusion in feed of chickens for fattening of 30 U β-mannanase/kg complete feed. This conclusion was extrapolated to all poultry for fattening.