diff --git a/NDTensors/src/lib/TypeParameterAccessors/test/test_basics.jl b/NDTensors/src/lib/TypeParameterAccessors/test/test_basics.jl
index 744a79af6e..7df288f3dd 100644
--- a/NDTensors/src/lib/TypeParameterAccessors/test/test_basics.jl
+++ b/NDTensors/src/lib/TypeParameterAccessors/test/test_basics.jl
@@ -15,6 +15,11 @@ using NDTensors.TypeParameterAccessors:
 include("utils/test_inferred.jl")
 @testset "TypeParameterAccessors basics" begin
   @testset "Get parameters" begin
+    @test_inferred type_parameter(AbstractArray{Float64}, 1) == Float64 wrapped = true
+    @test_inferred type_parameter(AbstractArray{Float64}, Position(1)) == Float64
+    @test_inferred type_parameter(AbstractArray{Float64}, eltype) == Float64
+    @test_inferred type_parameter(AbstractMatrix{Float64}, ndims) == 2
+
     @test_inferred type_parameter(Array{Float64}, 1) == Float64 wrapped = true
     @test_inferred type_parameter(Array{Float64}, Position(1)) == Float64
     @test_inferred type_parameter(Val{3}) == 3
diff --git a/NDTensors/test/ext/Project.toml b/NDTensors/test/ext/Project.toml
new file mode 100644
index 0000000000..df085f1623
--- /dev/null
+++ b/NDTensors/test/ext/Project.toml
@@ -0,0 +1,4 @@
+[deps]
+ITensors = "9136182c-28ba-11e9-034c-db9fb085ebd5"
+MappedArrays = "dbb5928d-eab1-5f90-85c2-b9b0edb7c900"
+NDTensors = "23ae76d9-e61a-49c4-8f12-3f1a16adf9cf"
diff --git a/NDTensors/test/ext/runtests.jl b/NDTensors/test/ext/runtests.jl
new file mode 100644
index 0000000000..d9f34a4f70
--- /dev/null
+++ b/NDTensors/test/ext/runtests.jl
@@ -0,0 +1,12 @@
+@eval module $(gensym())
+using Test: @testset
+@testset "$(@__DIR__)" begin
+  filenames = filter(readdir(@__DIR__)) do f
+    startswith("test_")(f) && endswith(".jl")(f)
+  end
+  @testset "Test $(@__DIR__)/$filename" for filename in filenames
+    println("Running $(@__DIR__)/$filename")
+    include(filename)
+  end
+end
+end
diff --git a/NDTensors/test/ext/test_mappedarraysext.jl b/NDTensors/test/ext/test_mappedarraysext.jl
new file mode 100644
index 0000000000..e322022f52
--- /dev/null
+++ b/NDTensors/test/ext/test_mappedarraysext.jl
@@ -0,0 +1,19 @@
+@eval module $(gensym())
+using ITensors: Index, itensor
+using MappedArrays: mappedarray
+using Test: @test, @testset
+f(i::Int...) = float(sum(iⱼ -> iⱼ^2, i))
+f(i::CartesianIndex) = f(Tuple(i)...)
+@testset "NDTensorsMappedArraysExt" begin
+  a = mappedarray(f, CartesianIndices((2, 2)))
+  b = copy(a)
+  i, j = Index.((2, 2))
+  ta = itensor(a, i, j)
+  tb = itensor(b, i, j)
+  @test ta ≈ tb
+  @test ta[i => 1, j => 2] ≈ tb[i => 1, j => 2]
+  @test 2 * ta ≈ 2 * tb
+  @test ta + ta ≈ tb + tb
+  @test ta * ta ≈ tb * tb
+end
+end
diff --git a/NDTensors/test/runtests.jl b/NDTensors/test/runtests.jl
index 2a05921e40..5a43e4ee90 100644
--- a/NDTensors/test/runtests.jl
+++ b/NDTensors/test/runtests.jl
@@ -7,7 +7,7 @@ using SafeTestsets: @safetestset
     filenames = filter(readdir(@__DIR__)) do f
       startswith("test_")(f) && endswith(".jl")(f)
     end
-    for dir in ["lib", "ITensors"]
+    for dir in ["lib", "ext", "ITensors"]
       push!(filenames, joinpath(dir, "runtests.jl"))
     end
     @testset "Test $(@__DIR__)/$filename" for filename in filenames