The results of simulations of the structure and internal motions of carbonomonoxymyoglobin (MbCO) at two different temperatures (325 and 80 K) are presented and compared with experimental data. Properties calculated from the 120 ps trajectory at 325 K are used as a reference in the analysis of the motion of the protein at 80 K. Three separate 80 K molecular dynamics trajectories were calculated; they were started with different coordinate sets from the 325 K simulation and the lower temperature was achieved by scaling the velocities. The simulations yield results for the structural changes between 325 and 80 K that are in general accord with those from X-ray data. Both the experimental and calculated radii of gyration, distances from the center of mass and main-chain difference distance matrices show that there is a significant but inhomogeneous shrinkage with decreasing temperature. For the atomic fluctuations, by contrast, the calculated temperature dependence is very different from the X-ray results; i.e. the calculated root-mean-square backbone fluctuations decrease to 0.11 A at 80 K from 0.51 A at 325 K, while the fluctuations obtained from the X-ray B factors go from 0.56 A at 260 K to 0.47 A at 80 K. The smaller temperature dependence of the B factors suggests that there is significant conformational disorder in MbCO crystals at lower temperatures. This is in accord with the simulation results, which show that the protein is trapped in restricted regions of conformational space at 80 K, while at 325 K a much larger region is accessible to the protein. Analysis of the fluctuations at 325 K and 80 K shows that the room temperature flexibility of the protein is determined by the mobility of the loop regions and by side-chain torsional motions (in accord with earlier simulation results), while the low temperature fluctuations involve motion within a single well. Examination of the calculated iron atom fluctuations and comparison with Mossbauer data show good agreement. It is found that the dominant contribution to the iron motion arises from heme sliding; motion of the iron relative to the heme are much smaller.