The alymphoplasia (aly) mutation of mice prevents the development of systemic lymph nodes and Peyer's patches. The mutant homozygotes (aly/aly) are partially deficient in both humoral and cell‐mediated immune functions. In the present study, we show that adult worm expulsion was slightly delayed and that T helper 2 (Th2)‐type responses were partially defective in aly/aly mice after infection with Trichinella spiralis. Male aly/aly and aly/+ mice (8‐weeks old) were infected with 400 muscle larvae. There was no difference in worm recovery between the two groups on day 5. However, worm recovery in aly/aly mice was significantly higher than that in aly/+ mice on day 14. Mucosal mast cells increased in number and peaked 14 days after infection in aly/+ mice. aly/aly mice were deficient in their mucosal mast cell response through out the primary infection. To examine the existence of mast cell precursors, aly/aly mice were treated with recombinant interleukin‐3 (rIL‐3) before infection. The mast cell response was poorly induced in aly/aly mice treated with rIL‐3. An immunoglobulin E (IgE) response was not detected in aly/aly mice during the course of infection. Serum IgG1 levels in aly/aly mice were significantly lower than that of aly/+. The serum IgG2a levels increased in both strains of mice. However, IgG2a production in aly/aly mice on day 14 was half as much as that in aly/+mice. Stimulation of splenic T cells in vitro with anti‐CD3 monoclonal antibody (mAb) showed that spleen cells from aly/+ mice on day 5 produced more IL‐4 than spleen cells from aly/aly mice. IL‐4 production from aly/aly mice on day 14 was half that from aly/+ mice. Interferon‐γ (IFN‐γ) was produced in both aly/aly and aly/+ mice on day 14. Proliferation assay showed that T cells of aly/aly mice responded poorly when cultured with antigen‐presenting cells. These results suggest that aly gene is needed for the induction of protective immunity and Th2 responses in mice infected with T. spiralis.