Cultivation of Laminariales is an expanding industry and intensive investigations have been conducted in order to optimize the entire production chain from spore to sporophyte. However, despite intense research within the field, literature on the very early development of kelp microstages is scarce. In that perspective, the aim of the present study was to clarify the role of different phosphorus (P) regimes on the very early development of Saccharina latissima microstages using spores with different seasonal origin and thereby contribute to the optimization of cultivation conditions for the early life stages of the species. Results indicated that spores of S. latissima had to obtain a given size before developing into multicellular filamentous gametophytes, and this size was reached at least 2 weeks earlier in spores originating from April than in spores originating from February. Nutrient enrichment (P1-P5) had a profound positive effect on spore and gametophyte growth and development, when compared to natural nutrients levels (C), whereas increasing P levels within the range 23-69 mu M phosphate-P did not induce any effect on growth of neither the spores nor the gametophytes with spores originating from April. Filamentous gametophytes appeared after 3 weeks cultivation of spores originating from April and although female gametophytes were recognizable approximately 1 week prior to male gametophytes, the female and males were found in the relation 1:1 in all nutrient treatments after 5 weeks cultivation.