Technical Abstract: During commercial cherry orchard pollination studies in California, we made observations that indicated possible side effects of tank mixtures of fungicides, surfactants/stickers and foliar fertilizers, on O. lignaria: nesting females completely disappeared from blooming orchards following early-morning sprays including Benlate, and Benlate and Captan; evening sprays that included Rovral and Orbit seemed to affect the pollinating activity of the females for several days. We therefore assessed for possible contact and oral toxicity effects of five fungicides on O. lignaria and A. mellifera in the laboratory. To determine chronic toxicity effects, survival of O. lignaria and A. mellifera exposed to high doses of the fungicides was compared to that of the untreated control over 7 days. For those products that showed acute toxicity effects at high doses, contact and oral LD50 values were determined (positive control: dimethoate). We also tested whether the size of the holding cage affected survival of A. mellifera. To our knowledge, this is the first complete contact and oral toxicity test on an Osmia species. Benlate alone seems to be safe to both bee species. Given the observed effects of Captan alone on female O. lignaria, the use of this fungicide should be avoided when O. lignaria are actively visiting the treatment area until the completion of additional semi-field and field tests. Captan alone may not affect pollinating activity of A. mellifera. However, Captan and Rovral have been reported to produce death in honey bee brood and could therefore weaken colony strength. The use of both Captan and Rovral should therefore be avoided while pollen and nectar are available on flowers. Trilogy alone could be a fungicide compatible with bees during the pollinating season, if direct exposure of bees is avoided. To prevent bees from consuming nectar contaminated with Orbit, we suggest not to apply this fungicide when nectar is available on flowers. In conclusion, with the exception of the Captan and O. lignaria combination, the tested products alone should be safe to both bee species, and pollinator function should not be affected if direct exposure is avoided. However, further investigations are needed on possible sublethal and/or synergistic effects with other fungicides and products that are commonly added to tank mixtures, on both O. lignaria and A. mellifera.