Last summer, a Detroit importer received a huge shipment of clothes that were described on shipping documents and labels as women's jogging suits.

But U.S. Customs Service officials inspecting the cargo became suspicious when the garments appeared to be much too large for women. Moreover, the styling appeared to be more masculine than feminine.

Under questioning, the importer insisted that the suits were properly labeled and that they would be sold in stores that cater to large and tall women.

But investigation of the importer's files turned up a cable message from the manufacturer to the effect that he could not obtain a quota for men's jogging suits entering the United States. Since he could not ship the cargo directly from Japan, he would send it through Taiwan, the cable said.

Anxious to avoid delay, the importer had replied that the manufacturer should ship the suits direct but label them as women's. The manufacturer cautioned that changing labels would involve additional costs which would have to be borne by the importer, but the two parties eventually came to terms on the new scheme.

As a result of the investigation, the merchandise was seized and civil and criminal charges are pending against the importer.

The case is one of hundreds that have been documented as evidence of fraudulent practices in the shipment of imports to this country. U.S. textile and apparel manufacturers contend that unfair trade practices, such as the Detroit caper, have exacerbated the textile imports problem.

U.S. law already provides domestic textile manufacturers with an array of protections that includes tariffs and quotas. But because these are selective -- aimed at certain products from certain countries -- they are easy to evade, industry experts say.

As a result, American importers and foreign exporters often join to mislabel clothes or fabrics, or to ship them along indirect routes that obscure their true origins.

"We seem to find more of that in textiles and steel imports than in any other area, but textiles is the largest area,", said a customs fraud specialist.

In fiscal year 1984, U.S. officials investigating such cases seized merchandise valued at more than $31 million -- a 300 percent increase compared with the previous year. This year "we will either equal or exceed that," a customs official estimated.

Tons of textile and apparel products entering the United States either arrive without the proper shipping documents or are mislabeled to hide fiber content, according to customs officials.

Customs has uncovered numerous schemes perpetrated by what Commissioner William Von Raab calls "textile bandits." At a meeting of textile industry officials this year, Von Raab, with exhibits as evidence, cited several examples of fraudulent schemes to increase textile imports to the United States:

*Sleeves of a jacket were imported through one American port and vests for the same garment were shipped to another. This deceptive practice grew out of an attempt to avoid import quota restrictions on jackets.

*Asian-made products were shipped to Lebanon and relabeled there as products of that country. An investigation by Customs led to seizures of more than $5 million worth of fake Jordache jeans.

*Customs intercepted a shipment of more than 6,000 pairs of men's pants from Hong Kong, bearing labels showing them to be 55 percent linen. A laboratory analysis showed them to be 55 percent cotton. Cotton is under a quota; linen isn't.