When vulnerability curves are used to assess the susceptibility of plants to drought, the water potential corresponding to 50% loss of conductivity is frequently used as a measure of susceptibility. However, this value does not distinguish between different patterns of conductivity loss, such as a rapid loss over a narrow water potential range versus a gradual decline in conductivity. We have applied an exponential sigmoidal equation to vulnerability curves obtained from four closely related Eucalyptus clones differing in drought tolerance. The coefficients of the equation were evaluated and statictically compared among the clones. If the air-seedling hypothesis of cavitation is accepted, these coefficients have biological relevance. One of the coefficients describes the position of the curve on the water potential axis and is equivalent to the water potential corresponding to 50% loss of conductivity. This coefficient could reflect the size of the largest pit pore per xylem vessel. Another coefficient is related to the slope of the conductivity loss, and could reflect the range in maximum pit pore size per vessel.