in Materials and methods. Transformation efficiencies of 3–18.5 × 10−6 were achieved, depending on the vector used (Table 2). This is more than an order of magnitude higher than previously reported frequencies of transformation of H. pluvialis. Out of 3×106 control cells plated onto selective medium, one spontaneously resistant mutant colony was obtained on TAP agar plates supplemented with norflurazon (Table 2). However, although this colony was spontaneously resistant to norflurazon, the partial pds sequence tested was identical to the WT strain, not exhibiting the GC to TG mutation (Fig. 5A and B). Resistance is probably due to a different mechanism. Transformation frequencies of short versus long and linear versus circular constructs were roughly the same. However, addition of ble transgene lowered transformation frequency. Improvement of transformation frequency was achieved by optimized cultivation and recovery procedures and the use of gold particles for bombardment instead of tungsten.