In single-particle tracking experiments, the diffusion coefficient D m
ay be measured from the trajectory of an individual particle in the ce
ll membrane. The statistical distribution of single-trajectory diffusi
on coefficients is examined by Monte Carlo calculations. The width of
this distribution may be useful as a measure of the heterogeneity of t
he membrane and as a test of models of hindered diffusion in the membr
ane. For some models, the distribution of the short-range diffusion co
efficient is much narrower than the observed distribution for proteins
diffusing in cell membranes. To aid in the analysis of single-particl
e tracking measurements, the distribution of D is examined for various
definitions of D and for various trajectory lengths.