Al-Bi and Al-In of exact monotectic composition were directionally solidified with solidification velocities ranging from 0.3 mu m/sec to 5 mu m/sec and temperature gradients from 2 K/mm to 6.5 K/mm. Three distinct microstuctures could be observed: fibers, strings of pearls or irregular arrays of second phase droplets. The transitions from fibers to strings of pearls is discussed theoretically and compared with the experimental results. The origin of this transition seems to be a Nichols and Mullins instability of the fibers during cooling from the monotectic to the eutectic temperature caused by surface perturbations driving volume diffusion of solute in the liquid fibers of the second phase.