Field and culture observations are presented, related to the morphology and life history of Chromastrum alariae (Jonsson) Papenfuss in Europe. The life history of C. alariae corresponds with the general pattern of the genus Chromastrum, showing a combination of a gametophyte with a unicellular base, 1-3 erect axes, a relatively simply built carposporophyte, including 3-4 carposporangia, and a tetrasporophyte with a septately germinating spore and a multicellular base bearing numerous erect axes. Gametophytes are monoecious. Pairs of spermatangia are formed on the cells of short branchlets, which bear on their top a carpogonium with a lateral trichogyne directed towards the spermatangia. Influence of temperature and daylength was tested in strains from the Isle of Man and Brittany. Some differences between these strains were observed. All stages in the life history have been detected in both the type material and herbarium material from Norway and Brittany. The affinities of the gametophyte generation of C. alariae, C. rhipidandrum (Rosenvmge) Papenfuss and C. kurogii (Lee et Lindstrom) nov. comb, are discussed.