The length of flames of wildland fires is a relative indicator of fireline intensity and an important index to fire effects and difficulty of control. A technique for measuring flame height and flame-tilt angle for the purpose of calculating flame length is described. Laboratory tests determined the feasibility of using cotton string treated with ammonium phosphate fertilizers to measure flame height. Ammonium phosphate treatments with an effective P2O5 equivalent greater than 10 percent by weight prevented the string from sustaining combustion above the zone of contact with flames. Treated strings were completely decomposed to a height 5 to 7 percent above the visually estimated average flame height. The strings were evaluated in nine prescribed fires in Douglas-fir logging residues. Operational use of the flame-height sensor is discussed.