ABSTRACT
In Costa Rica, Veronicellid slugs are the most important hosts for Angiostrongylus costaricencis. Apparently, these molluscs develop a resistant mechanism after being exposed to the infection. In naturally infected slugs, the higher infection rates were found in larger slugs, but they usually bear few larvae. larger number of larvae were found in medium sized molluscs. Experimental infection in laboratory breed slugs produced an amebocytic reaction around developing larvae; later, the formation of a fobrotic capsule is observed. When there is a second infection, cell reaction is stronger and the larvae show degenerativesigns. This cell-mediated resistant mechanism seems to explain why the biggest moluscs, although more probably exposed to infection, bear fewer larvae.