Mr Ruggli said the irregularities involved several million Swiss francs but gave no further details.
A spokesman for the bank said that, based on current knowledge, the arrest of Mr Heer, a former member of Zurich-based Rothschild Bank AG's management committee, would not lead to any losses for Rothschild clients.
Mr Heer was in charge of the bank's credit department until last July. At that time the bank said he had resigned for personal reasons.
A bank spokesman said in July that the bank had reviewed all its credit exposure at its recent annual general meeting and found some credits were not fully covered.
Rothschild Bank announced last month that it would need to transfer Sfr63.5m ( pounds 25m) from hidden reserves to cover depreciation, provisions and loan losses of Sfr99.8m at the end of March 1992, up from Sfr24.3m last year.
Rothschild AG earned net profits of Sfr24.4m in the year to March 1992 compared with Sfr24.0m the year before.