Microcoerivity is associated with the pinning forcce of a domain wall and has typically, but erroneously, been taken to be identical to bulk coercivity. Indeed, excluding certain limiting cases, it is shown that bulk coercivity is nonlinearly related to microcoercivity. Unfortunately, the precise functional relation between microoercivity and bulk coercivity requires making certain assumptions about the magnetic material of interest. This is illustrated by considering materials with either low or high defect concentration for which a rigid domain wall constraint is satisfied. Simplified theory for bulk coercivity versus temperature appears to compare very favorably with measured values for hydrothermally grown magnetites. ¿ American Geophysical Union 1990