The influence of a fire retardant (Firesorb, an acrylic-acrylamide copolymer) on the microbial community structure determined by phospholipid fatty acid (PLFA) analysis was examined under laboratory conditions using two different textured soils under pine forest. Firesorb was added to unheated and heated soil samples (350 degrees C for 10 min followed by reinoculation, to mimic a forest fire) at three levels of application (none, usual and three times the usual levels), and measurements were made after 12 weeks of incubation. The relative importance of the three factors considered on the PLFA profiles was as follows: soil heating >> soil texture approximate to Firesorb treatment. In the unheated soils, Firesorb had a larger effect than soil texture, while the opposite was found in the heated soils. Soil heating reduced the total PLFAs, while Firesorb tended to increase them in both the unheated and heated soils. Soil heating decreased the PLFAs indicative of gram-positive (G(+)) bacteria and tended to increase the fatty acids associated with gram-negative (G(-)) bacteria and, to a lesser extent, the PLFA 18:2 omega 6, considered to be predominantly of fungal origin. Firesorb treatment decreased the G(-)/G(+) bacteria ratio in the heated soils but tended to increase it in the unheated soils, the effect being dose dependent.