A decagonal quasicrystal with a periodicity of 1·6 nm along the tenfold axis, similar to that in Al─Pd and Al─Co, has been found in a rapidly solidified Al6Ni alloy containing a few per cent of silicon. In addition, a new decagonal phase has been found in Al4Ni which has a periodicity of 0·4 nm along the tenfold axis. This decagonal phase is different from those reported before in the aluminium–transition metal alloys. A new metastable crystalline phase Al9Ni2having the structure of Al9Co2has been found.