Sexual imprinting is one of several known non - genetical, yet social factors which influence mate preferences and might play a role in the evolution of novel traits. We introduced a red bill as a novel trait in a monomorphic estrildid finch, the Javanese mannikin Lonchura leucogastroides. We established three different imprinting groups in which the father only, the mother only or none of the parents had a red bill. After reaching maturity we tested the offspring in double choice tests for a response to birds of the opposite sex with a naturally coloured black bill or with an artificially coloured red bill. Neither males nor females showed a preference for potential mates with a red bill. Males and females raised by a red bill father showed even a strong rejection to conspecifics of the opposite sex with a red bill. This is in contrast to a previous imprinting study in the Javanese mannikin under similar conditions (Witte et al., 2000) in which males and females became sexually imprinted on conspecifics adorned with a red feather on the forehead. It seems that not all kinds of novel traits birds can be sexually imprinted on. We could show in the present study that the red bill is a meaningful trait in female mate choice, i.e. females responded to males with and without a red bill in a similar way as do females imprinted on natural type parents to males with and without other artificial adornments (Witte & Curio, 1999). We could confirm an interaction between the red bill and the natural attractiveness of males as found in a previous study (Witte & Curio, 1999). Our study opens up questions about what traits are really learned and why some traits are not learned during imprinting.