To investigate the behavioral mechanism of chemotaxis in Caenorhabditis ele
gans, we recorded the instantaneous position, speed, and turning rate of si
ngle worms as a function of time during chemotaxis in gradients of the attr
actants ammonium chloride or biotin. Analysis of turning rate showed that e
ach worm track could be divided into periods of smooth swimming (runs) and
periods of frequent turning (pirouettes). The initiation of pirouettes was
correlated with the rate of change of concentration (dC/dt) but not with ab
solute concentration. Pirouettes were most likely to occur when a worm was
heading down the gradient (dC/dt < 0) and least likely to occur when a worm
was heading up the gradient (dC/dt > 0). Further analysis revealed that th
e average direction of movement after a pirouette was up the gradient. Thes
e observations suggest that chemotaxis is produced by a series of pirouette
s that reorient the animal to the gradient. We tested this idea by imposing
the correlation between pirouettes and dC/dt on a stochastic point model o
f worm motion. The model exhibited chemotaxis behavior in a radial gradient
and also in a novel planar gradient. Thus, the pirouette model of C. elega
ns chemotaxis is sufficient and general.