In the present study we evaluated the musselspat feed MySpat formulated by INVE Technologies (Dendermonde, Belgium) in combination with small quantities of microalgae as a complete diet for young mussel seed Mytilus galloprovincialis (Lamarck 1819). Three different food levels were tested: a continuous algae supply over a period of 24 h of 150 cells μl−1(Control diet 1, C 1), 75 μl−1 (C 2) and 24 cells μl−1 (C 3). In three additional treatments C 2 was supplemented with 2.8% and C 3 with 2.8% and 4.3% MySpat respectively. Percentage was calculated on life weight (LW). Mussel spat belonging to treatments C 3 2.8%MySpat and C 3 4.3%MySpat gained almost twice as much weight as the mussels fed the nonsupplemented algae diet C 3. There was no significant difference between the two supplementation levels, indicating that a level of 2.8% was sufficient. The mussel spat that received the supplement MySpat grew as fast as the animals that received 75 cells μl−1 being 702% increase in wet weight (WW) in 3 wk, so the same result was obtained with only 1/3 of the algae. This is interesting when one considers that the mussel spat in the last week of the experiment received 95% dry weight (DW) formulated feed and only 5% DW algae. The growth was well balanced between shell growth and increase of tissue weight, because the organic matter content of the animals was equal to or even higher than the positive control animals. Mussel seed on the C 3 diet had a fatty acid methyl ester (FAME) content of 6.6 mg gDW−1, whereas this content quadrupled to 28.1 mg gDW−1 when 2.8% MySpat was given in addition to the algae diet, reaching levels even higher than for the positive control treatment. The fatty acid composition reflected the diet-composition, hereby proving the ingestion and assimilation of the diet. It is suggested that mussel seed regulate arachidonic acid (ARA) levels and keep the absolute amount in their tissues at 0.4 mg gDW−1.