A model for the 40 s subevent of the 1980 Irpinia earthquake is obtained from levelling data. An inverse method has been used to compute fault parameters, based on the computation of the probability density on strike and dip angles and midpoint fault location. The highest probability model consists of a fault almost parallel to the one associated to the first subevent. This fault dips about 80¿ SW, forming a small graben like structure with the main fault. Fault strike and dip parameters appear well constrained by levelling data, to within 10¿ (at 95% confidence level). It is noteworthy that seismological and accelerometric data were not able to well constrain the fracture mechanism of this subevent, which was the most controversial one in literature (Crosson et al., 1986; Westaway, 1987; Crosson et al., 1987; Westaway and Jackson, 1987; Bernard and Zollo, 1989). ¿ American Geophysical Union 1993