Nyko makes and sells a number of other accessories for the Wii, such as a charging station for Wii Remotes and a Wii carrying case.
Nintendo's case, filed in a Seattle court, states that Nyko's Kama Nunchuk "wholly appropriates the novel shape, design, overall appearance and even the colour and materials used in the Nintendo Nunchuk controller".
Nyko is accused of violating Nintendo's patents in the design of the device and trademarks in the naming of the product.
Nyko has yet to respond formally to the suit, but has told the media that it is looking into the accusation and has not intentionally violated Nintendo's intellectual property rights.