Benthic foraminifera from an intertidal mudflat (Bay of Bourgneuf, France) have been exposed to different types of oil-polluted seawater in an experimental laboratory setting. The aim of this experiment was to study the response of foraminiferal faunas from the intertidal zone to oil spills, as observed after the wreckage of the Erika oil tanker in December 1999 on the French Atlantic coast. In the course of the experiment a saturated seawater mix (SSW) and a water accommodated fraction of the oil (WAF) were on several occasions added to a part of the mesocosms, and the response of the foraminiferal assemblages was monitored during 3 months after the start of oil treatments. Several potential effects of oil-pollution were studied: 1) foraminiferal standing stocks, 2) anomalous growth patterns resulting in deformed or abnormal foraminiferal tests, and 3) changes in the taxonomical composition of the assemblage. The foraminiferal assemblages at the start of the experiment were dominantly composed of Haynesina germanica. The experimental results show a strong decline in density in all mesocosms (including the control mesocosms) over time. During the first 2 weeks of the experiment, this decline was stronger in the oil-treated mesocosms than in the control mesocosms. After the first 2 weeks, the replicates of the oil-treatments showed an extreme variability, whereas the control mesocosms showed less variability with densities steadily decreasing over time. In some of the oil-treated mesocosms, we observed strongly increased densities, indicative of reproductive events. In all other oil-treated mesocosms foraminiferal densities decreased more severely than in the control mesocosms. The different types of oil–seawater mixtures did not cause a significantly different response. After 3 months, some taxa that had not been encountered previously in the field samples or in the mesocosms (e.g., Textularia earlandi) appeared in the 63–125 μm fractions of the oil-treated mesocosms. We conclude that there is a dual response to oil-induced pollution: foraminiferal faunas may respond by a strongly increased mortality, and/or by an accelerated reproduction of some of the taxa.